You're going to have to file a ticket to have even a chance of anything being done. They're probably going to say they can't very well change the results of games that were played since that wouldn't be fair to the other players, but you can give it a go.
https://support.chess.com/customer/portal/emails/new
After that, give your nephews and nieces your password.
I don't think account sharing is allowed ?
As far as I know you're responsible for everything that is being done by your account unless you were maliciously hacked. However, it seems to me you allowed your sister access ?
As far as rating is concerned however, do not worry about it. If your rating was a correct indicator of your actual playing strength before your sister lowered it, then all you need to do is not think about the rating and just play a lot of games.
Then your rating will reflect your true playing strength again and thereby will have returned to about where it was before.
A likely story.
Next time I lose points, I'm going to blame my cat, and ask cc to reinstate my previous rating.
"A goat ate my rook pawn"
LMAO +1
A goat ate my rook pawn!
Well, eat the the goat, that's gonna teach it to leave your pawns alone.
Sometimes you're the goat, sometimes you're the rook.
I think the OP was just asking to have his account deactivated, not the results of games changed.
But, considering how few games you have played, it won't take you long to counter any effects caused by your sister.
I will play your sister.
Sometimes you're a good player, sometimes you're the sister.
I will play your sister.
Considering your hardcore history I don't think they will let you play the goat.
Don't even think about the sister.
Lets think of some devious way of getting back at his sister...
She got onto my account and messed my rating up.
Can a mod please deactivate this please. Thank you.