My forbidden lover
I'm currently a 30-something man, and I've been in love with someone who ISN'T my fiancée since I was in my early 20's, for over a decade. It's my older half-sister. We have been lovers since shortly after learning about each other's existence. She was already engaged and nearing 30 when we met, but couldn't help falling in love despite being siblings. We have kept our existing relationships only for show up to this day. Now I'm engaged and she's still married... and expecting a baby in her 40's (not mine, we double checked).
Only my best friend knows about us, and he's supportive. But it's very frustrating living this way: we are truly in love with each other despite everything, but we obviously can't show the world. While we are attracted to our respective couples, it is only when we are together that we feel fulfilled, complete. Almost every time we do it is amazing (and it also helps that, to be safe, we have always used "the back door" almost invariably). We even manage to take a week for ourselves and only ourselves, our yearly "honeymoons".
So perhaps we will never personally tell the world that we are siblings that recurrently make love to each other, but our bond is so strong that I thought it would be worth sharing even if it is anonymously. My own sister is my lover, world, deal with it.