ha, ha, welcome to my life. OK, so we don't live together but my partner is having a relationship with my sister and if it works out, down the track when her teenagers have left the nest, poly co-habitation is something my partner has always wanted to try. It's his ideal, don't know how Sis and I would cope.
I'm a writer too and thinking about writing something poly. Funny though that when I think about poly fiction I always think future sci-fi/fantasy, as if that is really the only place poly has widespread acceptance for readers. Maybe it's less threatening for people presented that way?
Back to your story. Neither my sister or I really identify as poly. Her relationship with my partner developed organically when she left her marriage and came to live close to us. She doesn't want a full time relationship so is happy with what she has with M (our hinge, my partner). If there was a poly/mono scale I think I would be somewhere in the middle and I am pursuing another relationship because I think it is a good way for me to cope with their relationship( I wouldn't be doing it but for M's polyamory). My bf is also mono so goodness knows what kind of letter identification you could place on all that. Oh I know it would look like a big tick, a v, elongated on one end
At present Sis and I haven't discussed her relationship with M. We are on good terms but I think it feels quite awkward for both of us. Neither of us wants to hurt the other or M so we are very careful about out actions. If you need any material questions answered feel free, it is quite an interesting dynamic.