If your MIL is up to the task, and she'll listen to your preferences (being an in-home caregiver, she's probably fine) then go for it. My parents keep my sister's kids all the time, and that started when they were babies. They are too far away to have me drop by kids off, but if they lived closer, I'd do it. My in-laws are 2 hours away. They've had our kids at age 3 for the first time because that's what they were comfortable with. My daughter hates going overnight without us because she is very much a mama's girl and she worries about me a lot (she's 6 now), but I think she needs the time with her grandparents. (Her cousins stay for a week at a time several times a year and LOVE it, so we know it's a good, safe place.) You can cultivate a much different relationship by being with your grandparents without your parents, and I really want my kids to have that.
So, if your MIL can handle a 2-year-old, I'd do it.