I'm going to sleep with my husband's brother
Why do you want to sleep with your husband's brother?
Because I'm in love with him.
I don't think it is a good idea to get involved with your brother in law. You could ruin the whole family dynamic and your marriage.
I thought no one was perfect.
Cheating on your spouse is not right. If they find out you could lose everything including your marriage, it is better to break up before hurting someone like that.