Why does she talk like this?

My sister is convinced she was abused when we were growing up. From my POV I had the best dad anyone could ask for. A little distant maybe.
Mom was volatile and started drinking heavily when I was a teenager.
But overall, from my POV we were well taken care of, spoiled even.
My sister has spent thousands on therapy.
She talks funny. She talks in a very formal, stilted manner.
She’ll talk about “Our mother” instead of just calling her “Mom.”
When she talks about our sister she calls her “Alexis….” that’s her name but we called her Lex or Lexy.
Dad, on the other hand, she calls “Daddy.” Weird.
I have more examples but hopefully that explains what I mean.