is this a friend problem or na

One of my closest friends has been hooking up with my brother. I’m not sure how far it’s gone and I honestly don’t wanna know. They were friends before I was friends with her, so I feel like I don’t have much of a say in what they do since I came into the picture later. But me and her got closer than her and my brother. I’m pretty chill and a “do what you want, I don’t care” kind of person, so their hooking up didn’t really bother me at first. But recently, it’s been weird. They’ve been sneaking around thinking I don’t know, and sometimes doing stuff when we’re all hanging out in a little group with some other friends. Like, you can do your thing, I don’t care, just not around me.

When she comes over, he’ll sometimes hang out with us and they’ll flirt and stuff, and I’ll just be there. Now I’m wondering, is this an issue? I feel like me and her friendship has this weird thing in it now because we can’t be totally open about everything because of her and my brother. I don’t wanna talk about their thing, but I want to be able to talk about everything with her, and I love hanging out with her. Is this a real friendship problem, should I do anything?