To some extent, she likes her various maladies and isn't ready to get rid of them.

Flames. Flames on the side of my face!

Dear Daughter in law: Exercise may make your MIL's pain/fatigue worse. Also, exercise for her may be having a shower!

Please don't pressure her/coerce her into being more active than she feels able to, you're not the one who has to cope with the pain/fatigue flare up.

Also, read up on the boom/bust cycle, or how over-exercise in chronic pain patients can lead to an increased level of disability...