So for clarification and maybe I missed it, but do you live with your in-laws? If not, then how are they in the loop with your lives? Like, how do they know when you hire a sitter etc?
I'm going to assume you do not live with your in-laws. If so, can you guys afford to move out?
With your MIL, this is all about setting correct boundaries in the relationship. Which starts with you not sharing every part of your life with her. That is what the husband is for. If she starts to probe, just keep it light and change the subject. Because she knows everything that is going on, that gives her power over you. So even if she is the only other adult interaction, it needs to stop.
Again, as others have said, start going to the kids groups. Go for walks to the park. Strike up conversations with other moms (or dads). You will meet parents who do have interests other than their kids.
What does your husband say about all this? Will he support not giving his mum all the information of your daily life details if she starts to probe? If so, then you are well ahead of the game. She can't put herself in your life if you don't let her. And what better way to put her off if she calls than to say "Sorry, I'm just about to take the kids to a play date." Let her mull that over for a bit.
My DH's mum has dedicated her life to the care and feeding of DH, to the detriment of her own life. But that was her choice. DH lets her come over to clean and to visit (up to 6 days at a time even tho she lives 20 Minutes away) because "it's all she has." Well, that is her choice. But he and I have decided together that we have to establish an appropriate boundary which is now we will go visit and take the kids over once every couple of weeks for dinners etc. Maybe have her over to watch the kids if we go out but then take her home the same night.
You and your husband have to decide what the appropriate boundary will be and then start to do it. Which means if she calls to pry, that you divert her by asking about her life or cutting the call short saying "everything is fine but I am (in the middle of cleaning, going on a play date, giving the kids baths) and we'll give you a call soon."
Then grab your kids and go for a walk to the park.
I can totally understand feeling isolated and alone. But you do have options and usually taking the first step is the hardest part. Get a book or magazine, head to those play dates and take the bull by the horns!
Now... if you live with them I have an entire different set of ideas. So let us know. :0)