I'm lucky to be married to someone caring and attractive, whom I love and who loves me back. My wife has a sister who's been married for several years, has kids, and seems happy. My wife is beautiful to me, but she doesn't have the confidence to pull off "sexy" like her sister.
A while back, my sister-in-law came over and we shared a bottle of vodka—my wife was pregnant so couldn't drink.
Hanging out with my sister-in-law and her husband is one of my favorite aspects of being a part of my wife's family.
Your confession was not reassuring, and you're lucky it didn't prompt your wife to suggest busting up your new family.
You also seem to be insinuating that you're insulted that your wife thinks her sister is not interested in you. You're an adult and a father, so stop acting like a horny teenager.
You gloss over the crucial point that your wife just had a baby.
That means that, for many months, she has not been as sexually available as usual, and even when she is, her flesh is distributed in strange and not totally alluring ways.
I'm not a cheating person, but I've also never been tempted before.