PLEASE keep this post anonymous

I am married for 9years now and my hubby mentioned some things about his sis in law when we first got married which I thought was weird because it seemed like he had a thing for her eg. he said she looks like britney spears and she is such a nice person, I also caught him looking at her on occasions...I was pregnant at that time and didn't think much of it...lately I have been thinking should I be worried?also how can one tell if they husband is cheating?