This almost happened to me. When I began dating my girlfriend, her mom was getting a divorce.
My father, who had divorced my mother years ago, took an interest in my girlfriends' mom. They became quite an item for a time, and we thought they might want to get married.
That would have left me dating my sister in law. I always joke privately with my wife that we married to aviod becoming brother and sister.
My family tree would have gone straight up, as per a Jeff Foxworthy joke.