Nieces hate me and say my behaviour was abusive.

Yes. This leapt out at me from the op They are saying I would shout and scream at them. I probably did get stressed at times but I would never hurt them.

People don’t have to be physically hurt to suffer abuse, and buying things does not make shouting and screaming okay. They obviously feel the behaviour they suffered has scarred them in some way and you are dismissing their feelings. No you shouldn’t have been put in the situation of caring for young children, but that doesn’t mean that what they say they experienced isn’t valid.

The injustice of your story is a terrible thing to contend with.
I think you should start to tackle this by writing it all down OP, as you have here.
See if that helps.

Separate it out.
Firstly about the abuse at the hands of your sibling who battered you and made you lie about your injuries. And any other abuse.
Then your actual babysitting memories, keeping it as factual as possible. How often, what you were required to do, how you coped practically.
Then what you thought about it at the time as a young teen being used and guilt tripped. Your feelings for yourself, the children in your care and for those who made you do it.
Then what you think about it now, what you missed out on and how that has shaped your life, all with the benefit of hindsight and with being a mum. What it cost you emotionally, socially and financially.
And finally, write about the current injustices from your nieces, the untruths/false memories, how that upsets you and how you would like to challenge them. Acknowledge how they could possibly have misconstrued some memories. Write about what you would like to say to them to put them straight. And what you would like to say to your mum and siblings for their deplorable behaviour.

Keep it all in a journal for yourself and add to it as thoughts and feelings crop up. Each time you finish writing something, close the journal with purpose, and with firm intent to leave the memories there and get on with your family life.

In the meantime, distance yourself as much as possible from those who are falsely blaming you. And those who knowingly or conveniently keep the truth a secret. Don’t be in their company if it’s too difficult . Focus on your own family.

One day, when you are feeling ready, you may feel the time is right to correct their false memories and misconceptions about their childhood and to confront your mum and siblings for allowing your nieces to openly blame you and spread hate about you.

This is really good advice. Try not to worry about your kids and them not being bothered with. I know from experience that they'll try to scapegoat your children too, because they see them as extensions of and occupying the same role as you. It's better that they're ignored.

@iCantStopppEatinggg I think it depends on whether you want to fix the situation or not?
IF you don’t - then concentrate on your family as many have said and go LC.
IF you do want to resolve because you’re sad and you’re feeling bad about it - then you will need an honest discussion. This will involve you properly “listening” and hearing what they have to say rather than your memory of events. You may well have been annoyed at your situation when you were a teen and shouting and taking it out on them because you were short of patience. You may not have realised the impact on them even if you didn’t mean it to stick with them.
Maybe if you can afford it - some kind of family therapist or someone to guide the discussions would be helpful. The key thing in these types of discussions is to properly listen, understand this how they really feel and let them explain. Even if you feel it’s unfair, and then apologise without saying I’m sorry BUT.
There may be an opportunity at some point to explain your point of view but not initially.
They may not accept your apology - it may take a while. but it would be a good first step.
BUT - if you don’t want to do this and feel it’s completely unjustified then definitely concentrate on what you can affect - your life, your kids and go LC x

Shouting happens when you're with young kids and it sounds like you had a lot of kids to deal with at any one time. Don't beat yourself up about it. Stick with the nice members of the family and leave the others to themselves. And, as others have said, don't get guilt tripped into elder care. It's your sisters' time to step up now. Leave it them and you concentrate on your own young family.

The best thing to do is simply distance yourself from the whole lot of them.

You should never have been used for free childcare for 6 children at such a young age. If you couldn’t always handle it - I think that’s par for the course.

Keep in touch with the nephews and niece who are supportive and just ignore everyone else.

You have your own family now. Focus on them and the future.

I could be way off with this thought, but are you of Asian heritage? Not that it excuses any of your family’s awful behaviour.

The fact that the other niece says they’ll look silly, means that everyone knows the score, and it’s only the two nieces that have an axe to grind.

Remind them that you stepped up when no one else did, that you sacrificed parties etc and if you made mistakes then you are sorry, but you did your best when you were much younger than they were. Also say that your memories are different, so you’ll have to agree to disagree over what happened, that you remember being kind to them when they were locked in the toilet etc.

Then concentrate on the nice members of the family, and ignore the nieces and their mothers who don’t actually sound that nice themselves.

I wouldnt undermine the memories and experiences that the neices have.

The screaming and shouting has obviously stuck with them more than the material things you got for them. It's wrong to deny that just because your nephews feel differently.

Your nieces are not wrong to feel how they do.

You shouldnt have had the parenting role forced on you. You were obviously not ready for it so yes it seems like you have made mistakes along the way and those are the bits that have stuck with the nieces.

Some parents do this too. They shout and scream then buy material things oblivious to the trauma and damage the shouting and screaming has caused. The children were never to blame.

Explain and apologise to them if they allow (and if you can genuinely try to understand where they are coming from ).

Maybe because they are tangible markers she can point to?

Op was used and given way too much responsibility and is now the convenient scapegoat.

Step away and stay away

Personally I wouldn't write a letter. I would just accept what's happened and distance myself. No pleading or begging or talking is going to change their mind . The OP is the family scapegoat and that label is going to be hard to shake off . The OP should take a leaf out of the nephews book and refuse to engage with the drama. Yes I think that one of her sisters is at the bottom of this trying to smear the OP because of the guilt she feels .

I'd probably distance myself/block any that are horrible to you. If anyone does have a go at you, point out you used to help and comfort your niece when their mum hit and locked them in the bathroom

I wonder if one of your sisters is at the bottom of this. Regrets are there to be mourned and grieved over but ultimately you can’t change anything except by drawing a line under it and not allowing your family to pollute any more
of your life. I would be writing an open letter to the family members involved and then I would be distancing myself. I would just use the letter to set the record straight as you recall and point out that things were not perfect as you were so young and but I wouldn’t back down.

The OP was only 16 when she was guilt tripped into looking after these kids . Of course there were going to be times when she was overwhelmed and screamed and shouted at them to get them to behave . We've all been there . None of us are perfect humans or perfect parents .

There are obviously two sides to every story and shouting and screaming is not a great way to be around children and often we only remember the negative things. Buying toys does not negate the bad moments either. Who knows what really went on but sounds like you very a child yourself really and maybe not equipped to provide childcare for so many kids. I would try and sit down with my nieces and ask them exactly their thoughts and your version too.

It seems as though they grew up to be the same kind of monsters that your older sisters are.

Don't engage with them on this matter - and go low contact. Focus on your kids and yourself.

Refuse to tolerate it and block them.

I imagine that they are taking it out on you for having feelings of being palmed off to you all the time instead of being with their own mother.

Rather than address an emotional baggage with their mother as to why her career was more important than bing with them when they were little they would rather take their angst out on you.

This.

Be kind to yourself @iCantStopppEatinggg you were taken advantage of from 16 by your family. You did your best. Move on and focus on your family.

Life is often unfair.

You have your own little family now and lovely children who need you. Expend your emotional energy on them rather than looking back.

With this information, is it possible that they are literally misremembering which aunt did these things to them?

Moving forward make sure you’re not the default carer for your parents generation.

Or indeed your nieces and nephew’s children.