Best in-law’s advice?

I work anywhere from 50-60 hours a week, I’m a grad student, and I deal with endometriosis on a daily basis (the pain uses my energy). All of that to say, I have to keep turning down my in-law’s requests to come have dinner with them on the occasional weekend, and it’s making them upset. They always choose the worst weeks for me and I just don’t have it in me. I feel bad, but should I? Any advice?