I imagine that none of you Wincest shippers has brothers or sisters and that’s why you don’t understand what is like to love a sibling. But if you do, then I hope someone writes fanfiction where you and your brother have sex.
Do you also go around telling people who ship canon friends that they don’t know what real friendship is? You do understand that most shipping of fanon pairings involves interpreting the subtext, don’t you? Shippers will take canon scenes and choose to give them a romantic spin, even though the canon is pretty much always intended to be platonic (familial love, friendship, etc.) This applies to all shippers, not just Wincest. We know it’s not canon, we’re just playing the “what if” game.
And really, each relationship, real or fictional, is unique. Saying you can’t ship Wincest because you have siblings and could never see yourself in a relationship with them is a bit like saying you can’t ship people who are close friends in canon because you also have close friends and can’t see yourself falling for them. I don’t know about you, but I don’t have to relate to fictional characters 100% before I can ship them.