Father in law

We have not kept it a secret with our parents that we were having problems with infertility. We told them in confidence that it will be between us only. My Father in law decided that he would talk to his side of the family about it...I was more than ticked off about it.
So, we want to tell our family around Thanksgiving time...how do I tell the family without having him tell everyone else? We really want to be the ones to announce...but he can't keep a secret about anything it's terrible.
Any suggestions???