I'm also a very introverted private person and when someone asks me 10,000 questions right in a row I find it very obnoxious because there are some things that I just want to keep to myself. My parents and DH's parents are fine and communicate with us within normal loving boundaries and I'm completely fine with that. I'm not pregnant & we're not TTC quite yet, so we're free from the crazy amounts of questions from close family members, as of right now. When that comes, I'll handle it by giving out all the information that I'm comfortable giving.
There is one extended family member, however, who is always asking questions (both on private matters and not). She's my aunt, and both DH and I get text messages, voicemails, and FB messages multiple times a day. When we travel the 400 miles to go visit her & other family members in that area, I love to sit and chat with her. I do love her, but I don't need 10 messages on FB in one day asking me if I have plans for the day, how DH is, how my new meds are making me feel, how work is going, etc... She's stopped asking when we're having kids, because we made it clear early in our marriage that we weren't going to talk about that with anyone.
Right now, she's asking normal & caring questions, but I'm asked them every day. If I don't respond to a FB message, she'll send it through a text message or the diet/fitness website I'm on that she also uses. When we don't respond for a few days, she gets mopey and sends us long long messages reminiscing on the days when we lived down there and would have such a grand-old-time. I feel bad because I know DH is weirded out by an extended family member having such an intense interest in our lives. He's not an introvert like I am, but he doesn't see the need for someone to know every detail of our lives. I saw someone in an earlier post use the word obsessed and it is definitely a word I would use for her. I love her dearly, but it's just too much.
I'm a huge believer in not judging, because I don't know what's going on in anyone else's life. I had a hard time putting words together to explain my aunt's intensity (still not sure I did it justice), so I understand if the OP did too when talking about her MIL. I don't really have much advice for you, unfortunately. Perhaps when the questions get too much, shift it off to your husband. "Oh, it was fantastic. DH was there too. He was excited to tell you about it." Obviously your MIL loves her son and would love to talk to him, so that's not like you're completely denying her any access into your life.
Re: MIL