Again. I know that's what they are going for, and if it was only a case of attraction to your family member's it would be no big deal. It's not like you can help finding your friend's sister or mom hot.
But she isn't just attracted to them, she's actively pursuing them despite this being upsetting to the MC. I can't really reconcile the fact she really likes the MC with how she treats him as a friend. I love you but I love ignoring your boundaries more is not the measure of a good friend. If her primary interest was for the MC, her primary concern would be protecting and growing this relationship, not putting it at risk for people she supposedly only likes because of their connection to the MC.
IMO the cuckquean role only works when that person is wholly devoted to their partner. If she's wingmanning the MC but also pursuing other interests herself, it's more like a casual sexual relationship where she doesn't feel possessive of the MC. She doesn't care as long as it's hot.