My sister has three kids and she's expecting her fourth. Her oldest (12f) isn't her husband's bio kid. They started dating when she was 5 and married when she was 8. Her other two are 4 and 2.5. My sister's oldest doesn't see her stepdad as her dad even though she has no relationship with her bio dad or his family. It was just my sister and her oldest for the first five years, first 7 if you count the years before her husband moved in.

The oldest has stated she doesn't have a specific reason for feeling like her stepdad isn't her dad. But that's bow she sees it. She's said more than once she will never call him dad and that she hates when people act like she should. That she can make up her own mind on that. My sister has a hard time with that but she was willing to let it go.

But from the time my sister got pregnant again there has been a HUGE dispute over the oldest saying half sibling. She never calls the other kids her siblings. It's always half. And she was never very excited about them. My sister blamed her daughter's BBF. The BFF had a very blended family and used half and step and only saw her full sibling as a sibling-sibling and the others were meh according to the BFF.

My sister and her daughter started fighting about the whole thing from that point onward. They fought it at home and in therapy.

My sister chose to cut her daughter off from the BFF. She got her moved to a different class and refused to let them hang out after school and when that wasn't enough she moved her daughter's school. But the girls still found a way to contact each other and my sister hates it.

My sister has also grounded her daughter, taken away all TV, phone and computer privileges. She has told her she is not okay with her making her siblings less than. My niece has said the truth is they're not siblings who have the same parents, they only have her the same and she can't change that. Which brought in the whole "Your stepdad has raised you for years now and you should accept him" as well as "Siblings are siblings regardless of how many parents they share or don't share".

My sister told me recently she found out the girls were meeting up secretly and my sister wanted to rage at the BFFs parents to make them get involved too. She said "that girl" has been a horrendous influence on her daughter and how much it's driving her crazy.

I told my sister I love her, I hate seeing her stressed and frustrated, but she's not making things better by acting like this. My sister asked what I meant and I told her she's destroying her relationship with her oldest and if she continues she'll be explaining to the other kids while their big sister is never around and never speaks to any of them. I said she might just need to accept that to her oldest the kids are her half siblings and her stepdad isn't her dad. I said maybe she'd get closer to them if she backed off and if not, then at least there'd be less fighting and they could be in a better place in the future.

My sister told me I didn't understand the relationship between a parent and their child if I felt she was destroying the relationship. She said she's parenting and trying to get more of a positive influence for her oldest.

AITA?