Your husband actually made a great start on setting boundaries by telling his mother frankly that her plans didn't work with your schedules. Clear! Direct! Simple! And the world didn't even end!
A technique I learned, as a life-long pushover, is to acknowledge what the other person wants, use the magic word AND, and add my firm, brief decision. Friendly, relaxed voice, not a hint of apology, and no reasons or excuses whatsoever.
It might sound like this: "Mom, I hear that you would like to come and stay with us for two weeks in February. That would be a fun / loving / helpful plan from your viewpoint. AND, NO, that simply will not work for us." Repeat as necessary, resisting your own habitual "need" to offer reasons.
From there, you can offer an alternative that would work for you. Perhaps two days on some specific weekend.
When other family members want to come, again, say no clearly, with the light comment that that does not work for you. If you start giving reasons or excuses, you open yourself up to argument or looking for other times when that excuse won't apply.
I had to practice this in front of a mirror endlessly before learning to say no to my mother's and sisters' eternal demands and expectations. Life has been easier since then, and I wish I knew how direct I could be 50 years earlier. And when I learned to say no clearly, my family learned to deal with that answer. I was and am surprised how little flack I get.
Good luck. You CAN do this!