Your wife's sister has never liked you very much and has always thought that you were basically a loser and not worthy of being married to her sister -- so from the very beginning of your marriage, she has enjoyed encouraging your wife to cuckold you quite regularly and she loves teasing you about being a cuckold.
"Yes -- it's OK with you? GREAT -- because I already told him I would, and I said that it was really sweet of him to say he'd like to start dating me again and that I'd really like that too. I have a strong feeling that you are really going to like being cuckolded -- and I know that I'm going to really enjoy cuckolding you with him."