I got a couple of good ones. First of all, I have 19 aunts and uncles so we have a lot of room for weird stories.
My Grandma was married to my grandpa for 25 years. When he passed away, my dad was twelve. A year later she married my grandpa's brother. So my dad's uncle became his stepfather and his cousins became his step sisters.
The next one is about one of my cousins. She met a really nice man and got married. At their wedding her mother met his uncle and they fell in love and got married six months later.
At my wedding, my cousin (on my mother's side) brought his nineteen year old stepdaughter. She and my cousin (from my dad's side) hit it off and dated seriously for almost a year. After they split up she pursued my brother for a while. But he thought that a second cousin through marriage was still a little too close.
I have another cousin who was high school a few years ago and decided to bring his girlfriend to a family dinner. It turns out she was the niece of one of my aunts but related through my aunt's mother's side, while my cousins was the same aunts nephew through her father's side. They broke up later that day.
Another one- My mother was the product of a relationship between my grandma and a married man. His wife and children never found out about my grandma and his other children (my mom is a twin). Years later my mom's half sister married my mom's cousin. They were married for twelve years and no one ever mentioned the relation. My grandfather had passed on by this time but his widow was still alive and no one thought it was worth hurting her over.
Writing this I realize how weird my family sounds.