I'm so sorry about your 2 MC's, to have to go into a chemist and pick up a prescription while random strangers sympathised must have been beyond grim.
I've had 3 MCs's-The second and third ones on-one knew but us about the PG or the MC at all, after having our private despair blabbed to all and sundry during the first one. (eg a random stranger I've never met coming up to me saying 'I can't believe they make you go through labour like that, can't they do a caesarian to get them out if they're dead?' in a petrol station queue in a town where my family live and I haven't for 20 years)
I'm afraid my experience with the first one is that some people in my family really like a good story to tell/drama to revel in, and they chose to place thaat above my need for privacy.
I learned from this never to tell them anything I'm not happy for random strangers in petrol station queues to know. Ever.
Recently we had quite a big issue happening at home which we hadn't tod them about until it was all resolved. One of my family took my DH aside and said they felt really hurt that they hadn't been told about it.
My DH said, 'i'm sorry we will never be able to trust you with any private infomation again after the way you chose to behave during Captain's MC. If you want other people to confide in you in future, you might want to rethink your behaviour'
which i thought was absolutely brilliant as its just the truth- not an accusation etc that could have caused a family argument, just plainly stated, our experience of their behaviour. They obviously had no comeback to this at all.
I'm sorry, it would be really nice for you to be able to confide in your MIL but clearly you can not. You are going to have to toughen up to protect yourself, she clearly puts her own need to tell the story above your need for privacy.