SD13 has always had a messed up sense of family relations. She calls her cousin on BMs side her brother, she calls her cousins (my niece and nephew) her nice and nephew too, and though she calls my kids her sisters she says she wants to be like an aunt to them.

She is old enough to be corrected because this is going to be confusing for all the little kids involved. 

The thing is SD is 13 and there are no other kids of her generation older than 3. She is a sort of middle generation and considers herself a junior aunt. Which does make some sense.

This wouldn't be a problem but my most hated SIL is the same, she is 11 years younger than my husband. This SIL is almost like a 2nd SD because she considers my husband is a father figure to her. She tries to manipulate him and get stuff out of him just like SD. She is super immature and likes to be a bad influence. She doesn't just fill SDs head with negativity regarding my DH and I, she actively teacher her how to be a manipulative mean girl. When they are together it's like 2 middle school girls, but one is 25 and can drink. SD loves her and idolizes her aunt. Argh, there are going to be two of them. 

I like that SD realizes she is going to be more like an aunt than a sister to my kids, but does it have to be that kind of aunt??