I'm also a writer. My list of things publishers have published includes an MFM poly novel, and two related short stories about a bisexual poly teenage boy. All of that's contemporary... Our contemporary society might not completely accept polyamory (and more than one reviewer ranted about the teen stories "promoting sexual promiscuity" even though the character didn't even kiss his boyfriend or girlfriend), but it exists in real life, so it exists in my books.
Nothing with siblings, though; that pushes my personal squick button as a reader and an author. It just isn't *my* thing.
On the other hand, in the erotica/erotic romance industry I'm seeing more menage stories involving siblings who are sharing a partner, and even some "twincest" books being published by smaller e-presses.
So to answer from a publishing perspective, it would depend on your market. There definitely *is* a market for the type of story it sounds like you're planning. It's still fairly small and niche, but it does exist. Some of your potential readers might have no real concept of polyamory; they just think the idea of two siblings sharing a partner is hot. But readers are readers--the royalty money spends the same regardless of why someone buys the book.
Me: 44, cis-het female, poly
Hubby: my husband, 42, monogamous
S2: my "person I like spending time with", male, 44, non-monogamous
Best Friend: male, 38, platonic; the one who keeps me sane through all this!
Guy: male, 43, former LDR boyfriend, maybe friend