Hi all
My brother's girlfriend (R)
 cheated on her EX Boyfriend (A)
 with my current boyfriend (B) 
Around four years ago. I was crushing on my current boyfriend, &amp; the three of us were lying in bed cuddling. I went to kiss B &amp; he told me how great a kisser I was. Knowing I liked him, R turned around anyway and kissed him. B said he isn't into me and that I'm just a good kisser.

This broke me. I was baffled &amp; hurt. It never seemed like R took full responsibility, let alone felt much guilt at all for cheating on her EX, albeit being honest with him and everyone. R made me super uncomfortable. B entered a long-term abusive relationship &amp; still crushed on R throughout.

Current day, I have done a ton of self-improvement, and B &amp; I are dating. 
My brother &amp; R have been together for three years. Their relationship is lacking at times since my brother suffers profoundly from mental illness.
The lack of emotional stability l

This leads me to my suspicion.

I told R that I had reconnected with B. Days later, I honestly thought I saw his name on her phone. 

I inquired R about it, and she quickly defended herself and showed them their last texts from 2020. We all typically spoke through Telegram, and she never pulled up the texts from that app.
I saw B had mutual snap friends, so I assume they were sexting. Even though R told me multiple times they never talked, I don't entirely buy it.

One thing struck:: 
So my brother suffers intrusive thoughts, and one of them is if R is cheating. 
R said
She "wants to reach out to B, but it would break my brother."

So she is already telling me something she fears he would overreact from.

Is it that I don't trust her, and I'm expecting old patterns? Or is this worth taking up with my boyfriend?
I also am being treated for BPD so maybe this is paranoia?
It would make sense since she's cheated before but also my mom gives her the car. R also finds B's actions with his ex from 2018/"unforgivable" and wants me to be careful. .

I just want to know