Why would she remove him as a facebook friend? iin?

So my friend is really upset over this and actually in tears because she thinks her boyfriend's sister hates her without even having met her.

This is what happened...
My friend is seeing this guy but it's a long distance relationship and she hasn't yet met his family but he's met hers. Her boyfriend Mike has been friends with his sister Marie on Facebook since 2010. Mike added my friend about 4 months ago on FB and the sister was fully aware because the status changed to in a relationship and she liked his status.

Anyway, the other day Marie went away on a trip for the weekend and while she was gone, he saw two new photos of her on FB with her friends. These were normal photos of her at her friend's house. But when she got back, she immediately deleted her brother Mike as a FB friend. My friend Marie noticed because she was going through his tagged photos and saw 15 of the recent ones of him from a wedding he went to that his sister took were missing. She asked him what happened and when he went to check, he saw his sister deleted him as a friend and he was really confused.

Apparently my friend's boyfriend and his sister get on well in daily life and talk every day. They aren't incredibly close but not distant by any means.

My friend is upset because she thinks she has something to do with this. I don't know why she thinks this but she keeps asking me what she should do and says she feels horrible. She asked him to ask his sister about it but he says he doesn't know how to.

Does this sound normal for the girl to delete her brother randomly like that? She goes away for two days and then poof, she deletes him from FB for the first time ever? She's 14 by the way and doesn't do drugs or drink so it's not like she'd be posting that and her mother and father are still on her friends list.