Welcome to my couples workshop, where people 18-80 of diverse cultures & orientations have come to explore their questions and find a path of heart. I'm unclear about whether these two couples you're writing about are all women, or whether they're heterosexual couples. There are somewhat different expectations about post-romantic friendships in some Lesbian communities than among typical heterosexual couples, yet musical groups have their own erotic energy among them, so there's quite a bit of competition for exclusive coupling.
It seems it would be pretty awkward to merely witness the harmony of their playing music together, because that is the voice of emotion, and it would not have led to her being forced out of the band if it had not been perceived as an invasive force. I have to say 1. I'm not qualified to state anything authoritatively about your situation, because I've never had to deal with that myself, nor carefully followed clients for whom that was the issue. But I think I would choose to be present for at least 2 jam sessions, so they couldn't give in to their erotic connection and see what happens. I'd have the assumption that it would be so uncomfortable to play together under your watchful eye, that they would have to discontinue it themselves, or perhaps their erotic energy would simmer down to the point that you could know it wouldn't gain momentum again.
What else should I consider?