So my SO and I dated 2011-beginning of 2013. We broke up on amicable terms, so there were no hard feelings. We got back together recently and things have been awesome.
BF has had this friend "Sarah" (not real name) since he was at least 7 since they grew up on the same street.

When BF and I first started dating, she really never seemed too fond of me. She wouldn't invite me to hang out with them, like ever. And if she was talking about me or BF told her that him and I were hanging out, Sarah would just go "oh her?" and never refer to me using my name.
I just brushed it off as jealously. I've always trusted my boyfriend because I trust him and I know he would never do anything with her. Like, ever.

Since BF and I have gotten back together, I have seen Sarah twice (BF sees Sarah now maybe once a month, if that. And rarely talks to her anymore after they had a big fight Summer 2015). The first time I was with BF and she was friendly enough but didn't engage in any conversation with me. The second was without BF and she seemed pretty fake, but I was super friendly.
I have always been super friendly to Sarah. I've never given her a reason to not like me.

But there is still the nagging feeling of her still not liking me. I recently just requested to follow her on Instagram on Sunday and it still says "requested" even though I know she has definitely had to have seen the request.
I'm not going to ask BF to drop her because he's still really close with Sarah's brother (22M).

But I really am not understanding why she just doesn't like me at all. She still doesn't invite me to any hang outs with her and BF and their mutual childhood friend "Trevor" (24M). But as of now, I have no desire to be around someone that clearly doesn't like me.

I've brought it up to BF and she goes "Sarah is crazy don't let her and her immaturity bother you"

How have you dealt with close friends of the opposite sex that are friends with your S/O that haven't been warm to you? I'm starting to think I'm just crazy and she actually likes me, but I'm just insecure.