You have an infant, fibromyalgia, and a full time teaching job. Any one of those three things would be reason to slow down, pay major attention to your needs and build in time to relax and recuperate a little. WIth the combination of all three of them, it's even more vital.
When you are in a stressful time in your life- like this one- pay really close attention to which people are rallying around you to help and support you, and which ones are putting more pressure on you to do more, be more, and make their lives easier. Your mother-in-law is in that category, yes- but so is your husband. He's got a front-row seat to see how stressful this is and knowing that this affects your health, and he's applying pressure to see his family more and make sure everything is "fair" as far as how much baby time the grandparents get.
So, the first thing to do is to have a heart-to-heart with your husband- and to be very upfront with how much you need to be a priority, and how upset you are that he's showing more concern for the grandparents' desire to see the baby than for your need to be rested and healthy. Also point out- schedules for how often to visit family need to be flexible for the stage of life you're in. Just from work schedules alone, there have been years that my husband and I could see family multiple times a week and years where once a month had to do. You need the flexibility in your own life to say "I'm not up to driving down to see my parents this weekend," or "I'm not up to a family visit this evening, I really need extra sleep." And while your extended family might not grant it graciously or might complain about it, your husband absolutely can- and absolutely should- intercede.
My honest advice is to sit down with your husband, look at the next month, and decide what you can whittle down. Not just family visits- look for ways to cut down or even skip some of the household chores, ways to get meals prepared and on the table quickly, everything. Is the babysitting your parents and his provide truly helpful, or does it create work in the form of needing to have the house clean or having to entertain them? And your parents sound very willing to make the drive- let them do the driving to see the baby instead of making the two-hour trip to go there. Streamline, relax, and let the focus go back to making the best life for your baby, self and husband, instead of the easiest life for your extended family.