wow - lots more replies, thanks everyone!
I'm not sure how to do multi-quotes but I'll try to clear things up if I remember it all
Someone mentioned it makes sense to have BIL ask his mum while visiting her. Maybe it did, but he phoned her specifically to ask her what the deal was, not during a visit. Maybe this changes things, I don't know.
Regarding the making sense he'd think of cancer - my husband's thinking is that he used the same phrase with others he's told and they got it - plus if it was cancer he'd have specified that rather than only mentioning the surgery/not having kids.
We're telling his entire family as they are a family full of gossips and no-one can keep a secret
They will all know about it whether we tell them or not and we figured we'd rather tell people ourselves as we see fit.
Someone, I can't remember who I'm afraid
, also mentioned not reading much concern in BIL's manner. I think this is part of what's getting me. Sure he may think we're overreacting but if it had been me I'd probably have thought, ok they're under stress right now so I'll give them a bit of a pass and say something like "OK sorry my actions upset you".
Thanks again for the advice/input guys and sorry if I've missed anyone's questions.