I think my SO's "cousin" is into him & their relationship makes me uncomfortable

My apologies for how long this is, but I really need advice. And yes, I already posted this in r/relationshipadvice but have gotten 0 responses, hence why I'm here.

My bf (32) & I (29) have been together 4 years & we're expecting our first child. We both had gotten out of long-term, toxic relationships before getting together. This is the healthiest relationship we've ever been in before & he's not the cheating type, which is a relief because my ex would cheat with anything with a pulse. But he has this pseudo cousin that makes me very uncomfortable.

The first 3 years of our relationship I met most of his family & heard of ones I hadn't met yet. But out of the blue, a female "cousin" (they aren't related at all, apparently they grew up as if they were cousins since their parents were close, but there is absolutely 0 relation) of his that he hasn't spoken to in years added him on Facebook & began messaging him CONSTANTLY. Like every single day, multiple times per day they would be talking all the time. She frequently vented to him about her husband that she was in the process of splitting up with. It really bothered me that I'd literally never even heard of this "relative" until 3 years in but my bf explained he hadn't even thought of her until she reached out to him.

His siblings seem to see her as a relative but it bothers me that she's so close with him & none of his other family in the same way. She talks to him about things you'd usually talk to another woman about. The other day she was venting about some guy she was messing around with & how she felt like she "was just a piece of ass to him", like stuff you normally wouldn't talk to a male relative about. None of the messages are inappropriate besides the odd semi-tmi stuff like that, but I just get the weirdest vibe from her. It feels like she doesn't actually see him as a cousin & she's prying for his attention a bit too much for my liking.

My boyfriend assures me he sees her as a blood relative & that he only wants me, but he's previously been completely blind to women that hit on him. This "cousin" reminds me of an ole friend of his who used to constantly message him all day every day & send pictures of herself (not nudes but several selfies getting him to say stuff like, "You have an amazing smile", just really attention-seeking) to him fishing for compliments. I made it clear I was not okay with it & that she needed to stop. He insisted she was just being platonic & that she had no interest in him, but I'm a woman too, I know exactly what she was doing. This "cousin" gives me similar vibes & I can't explain why.

I admit I do consider myself to have "relationship PTSD" where as I'm so used to being cheated on & abused that I'm constantly worried it'll happen even though I'm now in a completely healthy, faithful relationship. He's never been inappropriate with another person but like I said before, he's completely oblivious to women who are into him.

I initially thought this girl was a blood relative & I still felt very strange about how often she talks to him & the very personal things she discussed, but one day in passing it was revealed that they aren't actually related at all which increased my paranoia. I have a couple friends that I see as cousins because we grew up with close parents so I get that aspect, but there's no way in fucking hell I'd mention my sex life to them & I don't message them all the damn time. I can't figure out if I'm just severely damaged from my ex or if this girl really has shady intentions. She has made no effort to befriend me at all except to add me on Facebook & messaged me once asking about a previous TV experience of mine. Other than that she doesn't speak to me yet messages my boyfriend every single day. She's not close with other members of his family like she is with him.

I absolutely don't want to be that psycho girlfriend who sees every female in her boyfriend's life as a threat, but something just doesn't seem right about this girl. My boyfriend doesn't understand my concerns at all & I feel dumb when I bring it up because he sees her as family but I'm convinced she has feelings for him, & I don't like it. Am I crazy or are my instincts on to something?