How would everyone know? Bobby hasn't admitted to anyone else's involvement and as far as I can remember Ian and Jane haven't been publicly implicated in any way. Maybe I've missed something?
I'm sure that will be Vincent's initial reaction, although he might possibly come around to the idea. However, the fact remains they're not brother and sister and weren't even brought up together. I'm sure there are real life couples who grew up treating each other as siblings but who later fell in love, married and had kids.