change their relationship (sister-in-law)

Tom wondered how changing their relationship might change Sarah's personality. He thought about his brother and the kind of girls he always ended up with. Jonathan was five years younger than Tom and drove Tom's family crazy with the girls he dated. He has been married once but they divorced a few years ago when he found she had been sleeping around with multiple people throughout their marriage.

Unfortunately Jonathan had a type: younger, shallow, vain women who were more interested in hooking up and having a good time than settling down. Since the failed marriage there had been a string of such girls, though Jonathan didn't really seem to mind too much anymore.

"How would it be if Sarah was like that?" Tom thought. And he felt a little thrill at the idea of perhaps cuckolding his brother, having a secret affair right under the noses of his family. He decided on a wish - "I wish Sarah was my sister-in-law whom I'm having sex with."