I have a different opinion to most people here. While I can see how this could be upsetting for you, I wouldn't let it end relationships or destroy your opinion of your MIL.

I had a similar-ish situation happen to me. My parents had a big secret which my brother accidentally discovered. They then sat down and decided not to tell me until the last possible moment before it came out anyway. It was several months before I found out. They sent me off to counselling when I first found out and the first thing the counsellor asked me was was I upset about being the last to know. Honesty no. It really didn't bother me. It still doesn't.

I can understand where they were coming from. They didn't know how I would react (like you I have no mental health issues, sickness, breakdowns etc), but it was pretty big news. Part of it was wanting to protect me too.

I'm sure that at no point did your family sit down and say "how can we betray thr0waway488?" or "how can we hurt her with this information?" Instead they probably thought about somehow protecting you, as others have said, from a potential scam, or damaging your view of your father (they may have been thinking about how they would react to hearing they had a previously unknown sibling and this influenced their view - particularly your MIL and mum). Maybe they really did want to let your dad tell you? I can definitely understand Miranda going to your mum first about this. It could have been that Miranda thought that your mum had no idea and didn't know how she would take the news, or that your mum might be ok with a possible potential child, but confirmation would be upsetting. Maybe Miranda wanted to find out more information.

I would let your family know that you weren't happy to be left out for so long and then ask them not to do it again. Your fiance is very apologetic so its unlikely he'll make a similar mistake again.