Is it normal to fantasize about your sister-in-law?
Mostly.Normal for most people
Yes, fantasies about someone off-limits can happen and don’t automatically mean you want to act on them. What matters is keeping respectful boundaries in real life.
Yes, it can happen. Your brain may latch onto someone because they’re attractive, familiar, or simply forbidden-feeling; a fantasy by itself doesn’t say much about your character or what you truly want.
People differ on whether they’d want to know about a partner having this kind of fantasy, but most would draw a clear line at flirting, pursuing it, or creating secrecy that could hurt the family. You can let the thought pass without feeding it or turning it into a real-life situation.