I’m very, very sure!
I know he wants that...
She's right. You DID know the reason -- or, at least, you were pretty sure that was the reason. And since there's nothing you can do about it, you know you are just going to have to accept that being cuckolded is a part of your relationship with her.
Your sister-in-law was amused to see how totally defeated you looked as the meaning of what she was telling you sank in. And she looked forward to telling you wife about how much this news was devastating you emotionally, even to the extent that you had trouble keeping yourself from sobbing -- because she knew how pleased your wife would be to hear about your emotional distress.