A very simple Love Trope. Bob falls in love with Alice, but Alice does not reciprocate. Bob eventually marries her sister Carol. Look out for when this overlaps with Double In-Law Marriage. This can entail a Betty and Veronica situation, where a quiet Betty whom he would never have noticed in the ordinary run of things comes to his attention when his prolonged wooing of Veronica means he sees a lot of her and so comes to notice her profound but not paraded virtues. See also Does He Have a Brother?. Love Father, Love Son is similar, but with a generational difference.