I prefer her
When i was 15 and she was 17, my sister and i got drunk and ended up having sex with each other. it was an accident but neither of us could hide that we enjoyed it, so we continued to do so every now and then, just for fun or when either one of us had an itch to scratch, if you get my drift. this continued well into adulthood, but we never made this sort of relationship public for obvious reasons. so far we haven't been discovered and, as far as we know, no one has ever suspected us.
thing is, she now has a husband, i have a wife, and despite sometimes feeling bad about doing them wrong, we still hook up a lot (i am now 33, and she's 35). she makes love to his husband, and i make love to my wife, but we have never made love to each other. we mostly fuck in the rawest way we can when we need actual fun and pleasure, to the point the bed ends up reduced to a wet mess of our fluids. i do love my wife, but fucking my sister, as wrong as it is, is what fulfills me sexually, and that's the same for her (she has this ability to squirt even from ass fucks that i just can't get enough of.)