Future mother-in-law invited herself to my family's Thanksgiving. How do I tell her

[Lisa]

she can't come? FH and I are spending Thanksgiving with my family this year-FMIL was going to spend the day with her sister, but her sister will now be visiting other family, which means FMIL will be alone-she did get offers from members of her church to spend the day with them, but she turned them down FMIL told us last night, she will be coming to spend the holiday with me and my family and FH. Number one, that was extremely rude of her to just invite herself like that, two, it won't work as my family does not like FMIL, but more importantly, she can't come because my brother is coming home from Afghanistan for the holiday-we have not seen him in over a year and this is a very special, very private time for myself and my family (FH is very close to my brother-my brother introduced FH and I). Also, we were advised by military personnel to keep the gathering very small and limited to immediate family only, so as not to stress my brother. FH told his mom that-she is VERY sensitive to being left out of anything-and she immediately said my family thinks "we are too good for her" and accused me of trying to "cause trouble by keeping the families seperate." What do I do now?
To all those who think I am lying, I am not. PTSD is something that is very real. My brother's health must take precedence! I am shocked that so many people do not see that. I am guessing you have no military people in your family circle.
FMIL is not coming. FH is going to talk to her. It is just not advisable to have her there. She has never met brother and she is not his family.