Weird bridesmaids situation

So I’m in an awkward place with choosing this one girl to be my bridesmaid.
So my fiancé’s family has this girl that lives with them. She came from a rough situation and his parents took her in when she was like 16. She she wasn’t adopted or related.
She just got married and I wasn’t in the wedding (fyi this isn’t me being petty for her not putting me in her wedding, we don’t really know each other so I wasn’t mad at all) but my fiancé was because she thinks of him as her brother (but he doesn’t really feel the same way). Anyway, it’s my turn to get married and I don’t want to have her as a bridesmaid because I never really have hung out with her, we’re not close and I don’t feel obligated because she isn’t actually my fiancé’s sister. But here is where things Are awkward, my mother in law is going to expect me to have her as a bridesmaid because she has lived with the family and considers her family and as a “child”. But none of the other siblings really think of her that way. I don’t wanna piss anyone off but I feel like I shouldn’t have to have her in the wedding. I wasn’t in hers because she doesn’t see us as that close but my mother in law will want me to anyway. I already have 6 bridesmaids picked out and don’t really want anymore but I don’t want my mother in law mad because I didn’t put the “adopted” sister in the wedding.Side note: the girl didn’t really live at the house that much after high school, she went to college and didn’t really come back to often unless it was holiday or something. So I really never got to know her that well when I met my fiancé What do y’all think? And sorry this is kinda long and complicated