And here I understand the origin of why you thought Ayano told HIS sister about the affair.. NOPE! That is AYANO'S sister! Her OWN sister. She told her OWN sister that she had an affair, but DIDN'T tell her that it was with a woman, then Wataru sent her to the bar to discover that his sister had an affair with a woman. So... that's what happened.
I might have made a mistake about who Kaede was but the point still stand, if Ayano didn't talk to her sister about the affair, she wouldn't have ask her brother in law about the matter and i didn't he will have bring it
but he does know that Akari's last name DOES, so he sent her there expecting her to find out that the person Ayano cheated on him with was a woman...
How ? Saying the person's name start with a H is not the same as saying she is a woman. I mean, she even think it's a man atfirst and just bring the pieces together by thinking it throught.
Wataru's attitude is mostly due about how casual Ayano has announced it to him and how she doesn't seem to be bothered and also well,imagine you're married from some years with a woman and suddendly she come and said "Oh yeah i cheated on you with a girl." Most of peoples will be freaked out and didn't think rationally. He just don't know how to react in this situation.