How Do We Tell Her?

Ok, here is my situation. I have always wanted to know more about my bfamily, but was always concerned with my amom''s feelings. She has always been open in telling my brother and I that we were adopted and what our birth names were, but other than that she has made it quite clear that she did not want us looking for our birth families. Not that she would outright say "no", but she would get extremely upset the couple of times it was brought up. We would feel so guilty we would back down and not mention it again. I am a 30-something adult now and have an overpowering desire to find my bfamily. Mostly because I have children of my own now I feel the need to know medical background and family resemblance. But more importantly I can't imagine what it must be like to give a child up for adoption and never know what happened to him/her. I want my bmom to kow that I love her for making the right choice for me and that I turned out OK. I would love nothing more than to hug her and thank her for making I am sure, the most difficult decision of her life. But this just starts my story. I have signed up for registers on the internet that I check regularily and one day I stumbled on my brother's bmom. I wrote her and talked to my brother, who at first would have no part of it. Upon probing him further I discovered that in 2000 his bmom had gone through ADR and they contacted David. They asked if he would like to have contact with his bmom and he said yes. When he told our amom about it she freaked out and started crying, so when they called back he said he wanted no further contact with his bmom. When he told me this I was stunned that our amom would guilt him out of having a relationship with his bmom. After all he was in his late 20's by this point. What did she think he would do?Anyway, I have become very active in trying to get BIll 183 passed in Ontario, since we are about the only province left with closed adoption records. I have gotten petitions signed and have talked about this to everyone I know, except my amom. I have the full support of the rest of my afamily. They think we have a right to know and a right to a relationship with our bfamilies. So, my amom and abrother are coming to my house this coming Fri. for a family gathering on the Sat. My question is since we are going to tell her about David's bmom and that I am looking for mine, when do we do it? Fri night, Sat before the family dinner, or Sun. when she is getting ready to go home? I don't want to leave my brother "holding the bag" so to speak because he lives with her and has a 4 hr car ride home, so when do you think would be the best time?