I won't say there is a "saying" out there about this, but rather maybe more of a "sentiment"?  That if we ever get put in the position of control over our abusers, that we should do better than them.  And mostly, I do.  I am a billion times better than her to her than she was ever to me (and she still is bad to me).  I treat her with dignity, and actually take care of her when she's sick or at risk of being sick or hurting herself.  I take her seriously when she complains about pains or aches or symptoms.  She never did that for me.  But there are just some things I absolutely won't compromise on.  

1.  I won't waste money on her for stupid shit she wants (things she needs, yes, but not silly things she wants and doesn't need at all).  I will a little bit, now and then, because I am not a monster.  But not every single day, as she expects me to.  I won't give her expensive food or flavorings, because she can't taste them anyways.  I buy her cheap ranch sauce, not the expensive kind.  Things like that.  She did the same with me as a child, so why should I change for her?  But not only that, she can't freaking taste it!  Nor does she appreciate it when I do.  So, no.  I refuse to waste money on her for things like that.  And she doesn't complain about it.  So it's a win-win.  Rule: no wasting money on expensive food items (or frivolous bullshit) that aren't actually better (esp. when you can't taste them anyways).

2.  When I was a kid, I got clothes when I needed them.  I didn't get to shop all year-round for them, whenever I liked.  And I was never bought new clothes, only used.  So, that's what she gets.  Well, not really, I buy her new jeans every few months, but only because she ruins them regularly.  I can't depend on resale shops having her size, so I buy them off Amazon Wardrobe, try before you buy.  But before I took over everything in our house, she bought new clothes at least once a week.  Her HUGE closet is still filled with shirts, from front to back.  It's insane.  At first, she fought me on this.  HARD.  But now she's accepted it, I think.  Who knows with her.  But I don't let her shop, period.  I buy everything for her.  If I let her shop, she'd spend all our money in a week.  She's a shopping addict.  No buying clothes for fun until she gets rid of most of what she has already.  Rule: no wasting money on frivolous clothes (or anything else) shopping.

3.  I sometimes feed her exactly the same shitty meals she fed me as a child.  Not all the time.  Just sometimes.  They're cheap, easy to make, and I waste no time putting them together.  Growing up, we ate out of a box.  Period.  That was the extent of her gourmet cooking.  If it didn't say En-Cor, we didn't eat it.  I don't always feed her that crap, but I don't always feel like cooking.  Rule: if it was good enough back in the 80's to feed her family, it's good enough now.

4.  I treat her like a child.  Which is because she acts like a child.  I've told her numerous times I am her mother now.  And she's my kid.  It's not an insult.  But the tables have turned and now it's time for me to be in charge.  She was left to her own devices for FAR too long.  And because of that, she put herself in harm's way a thousand times over.  And now I am stuck cleaning up her messes.  So it's my way or the highway with her ( thought know I don't treat my family this way).  Rule: she may not like my rules, but if she wishes to live my house, she will obey them (or else it's to Shady Pines with ya, Ma!).  

5.  If she asks me too many times for something she's not allowed to have or for something I just can't get yet, I will put it off longer just to piss her off (know when I mean she's hounding me or being pissy).  Well, unless she can't have it, then it will never come.   But if she can't have something (like milkshakes, since she's lactose intolerant), I will make her an equivalent (like a frozen banana shake) that tastes the same.  But if she's a bitch about it, she gets nothing.  I don't allow her to be nasty to me.  I am not understanding of her shitty moods.  She never gave me one ounce of understanding as a child, so she doesn't get much from me, not when she's moody.  Also, if I do something nice for her and she complains?  She doesn't get it again.   Rule: if she can't ask nicely, she gets nothing.

6.  If she doesn't do as I say, I make it so she can't defy me again.  I have rules to keep her safe.  She's a literal idiot and will do things on purpose to hurt herself to get attention.  "I was standing on my counter today today to clean the top cabinets!" she once told her doctor in front of me.  "Shay saw me and walked out of the room.  What kind of daughter would leave their handicapped mother to do something like that??"  Me.  That's who.  This was years ago.  But she was still handicapped and still dizzy and still supposed to be walking with a cane.  And I caught her on her fucking counter.  So I just turned around and said "Good luck with that."  I was not there to save her.  She chose to put herself in that situation.  I did not live with her then.  But if I caught her doing that now?  I would remove all the chairs from the house and lock them away.  I already took away all the stepladders for this reason.  If I tell her once to stop doing something stupid?  And she does it again?  I remove that something (or lock it up, etc.) so she can't defy me again.  I was not this kind of mother with my kids.  I wasn't strict with them.  But I am beyond strict with her.  Because I have to be (otherwise she'll weasel her way back into doing whatever it is).  The thing is, she wasn't strict with me.  It was quite the opposite.  She was downright neglectful (minus the times she made arbitrary rules for no reason that didn't make any sense other than just to flex her "power over me" muscles).  But I refuse to be a neglectful caregiver to an elderly person who's hell bent on putting herself in the hospital (she broke a rib making a stupid choice in 2020, and broke her wrist also making a stupid choice in 2019) or other people (like, me taking away her right to drive when she refused to not drive down the middle of the road).  Like I said, I am her mother now.  So, my job is to keep her safe.  Even from her stupid self.  Though this rule also applies to rules I have to keep myself and family safe, too, whether they are physical things or personal boundaries. Rule: do as I say, or else I will take away your choice so you have to do as I say.  

I am sure there are more I could add here. But this is how I was treated as a kid (mostly).  And I see no reason to be different with her.  Thy say, if you end up in the position to have control over those that hurt you to be kinder.  Yes, I do agree with that.  Always be kind.  But don't be a freaking pushover.  Assert yourself.  Get some balls and flex them when needed.  I was born without balls (I am a girl, heh).  And my mother kept my balls at bay for all of my life until 2021 when I found them again (she was in a home for several months and I took over everything).  She thought she was the ONLY one with balls.  But as it turns out, my balls are not only bigger than hers, they are smarter, wiser, kinder, and just all-around better.  And did I mention they are soooo much bigger??  Yeah.  They're huge now.  I get to heal myself with my big 'ol balls and asserting them over the woman who has abused me up until...well, she's still abusing me.  But she has no power anymore.  At least none that she knows about, anyways.  

So, no, I don't always choose to do better than her.  I choose, at times, to give her doses of her own medicine.  But not in any way that will hurt her, unlike how she "parented" me.  There were plenty of times I could have and have been seriously hurt in her care (she recently just physically assaulted me, on Thanksgiving weekend of this past year).  So while I choose kinder ways to repay her, it's still healing to not always "do my best" with her.  Or maybe I am doing my best.  When I give her shitty dinners, I usually don't feel well or I am too tired.  Or sometimes it's "just because I want to".  So, her being fed, period, is my best (and she's fed every single day LOL).  But sometimes I don't do nice things for her when she's an asshole to me.  And that feels good.  It feels good to not have to bow down to the bitchy-ass overlord that masquerades as my mother.  I can say no sometimes.  It's quite empowering.  I suggest you try it.  It may change your life.  It did for me.  And still does, every single day.  

Okay, off to go read my new book I bought.  And tomorrow, I will be back at the grind, known as being my mother's caretaker.  Fun!