I agree to keep the high road. I'm sure you're not the only one sick of her behavior.
I have a relative who drives me crazy. When I have to deal with her, I am polite but don't go out of my way. If she comes up in conversation, I might say things like, "Oh, but you know how she likes to be in charge and if it's that important to her, she should just do it herself." Smile. Blink. Move on.
She "stole" Thanksgiving from me and I decided after that I wasn't putting my family through the hassle of driving there when we could just as easily host my in-laws here. Etc.
As for her FB digs, ignore her. If you don't defriend her, filter her so that she sees only what you want her to see. I do that with people. They don't get ALL the posts, or videos or pictures. They only get a few, if I feel like it. One might say, "Why keep them on your list?" Because of family dynamics, really. I have a cousin I blocked entirely.
Anyway, if you do defriend her, don't even say anything. Just do it and let her figure it out.
I think that your SIL is very immature and insecure. Anybody who need that much attention all the time and that much recognition has his or her own issues. Just keep that in mind. I think she WANTS it to be about your friend because then it's not HER behavior, it's you. Victim mentality.
I would tell her, if she says it again, "No, I don't like you for your own behavior. It has nothing to do with my friend." And move along.
I try to bear in mind with my relatives that it's no secret I don't like my cousin and haven't since we were kids. I don't seek her out at events. I ignore her. I let her behavior speak for herself - like when she came barreling over at our other cousin's wedding and shoved herself between other guests because she didn't like where she was sitting (she had not RSVPd so she was lucky to get a seat at all). There's another one who likes to act like I'm not an adult and I subtly remind her that I've had kids almost as long as she has, etc. I do have a clue. Thanks.