I can TOTALLY understand what it feels like when someone tries to tell you what you feel isn't real. It is real and you know it! I am one of those, like you, that look too good to be sick. My own mother has a hard time with coming to terms with my MS. She doesn't want to hear me talk about my future as if there is anything wrong now. I fight fatigue till I have almost nothing left to fight with. Yet I can be at my mothers house because I am too tired to drive to my home and she wants me to go to the grocery store with her so I can lift the groceries for her after shopping, or do the dishes, or take out the trash! This all seems resonable to a "normal" healthy young person to do for their own mother, so I feel guilty when I can't. She thinks that I am exaggerating. I am not, I assure you.
I am sure your mother-in-law loves you very much. But she may never understand what you feel. I find myself talking with my mother about how MS effects my life all the time so she won't think I am making excuses when she asks me to do things. I also do as much as I can for her on days that are good... She still thinks I am exxagerating when I am not having a good day...
Good luck with you MIL and just know that you aren't alone. I am sure everyone here can think of at least one close person in their life that doesn't want to deal with how this chronic illness is affecting their life.
Diagnosed with Multiple Sclerosis September 2003 currently taking Copaxone.