Yes, MG runs in families, but rarely. I have MG and so do both of my brothers. We have a super-rare form of congential myasthenia, which is thought by some to be a different disease from garden variety MG because it is genetic. However there are other instances of familial MG. In my many, many years of discussing MG with other MG patients I have encountered more than a few folks whose parents or children have autoimmune MG.

It is time to play hardball with your mother and insist that she see a specialist, probably an neurologist or at least an Ophtho-neurologist. Otherwise, guess who's going to be her caregiver. Tests for MG are uncomfortable, but bearable. The great thing about MG is that it is treatable. Most patients eventually enjoy some acceptable level of strength with proper medication. It just takes a while to figure that out. The longer your mom waits for a diagnosis, the longer it will take her to get back up to par. Good luck.
Set your Google Alerts to "congenital myasthenic syndrome" and "familial myathenia," and you'll find evidence. Not exciting reading, but we're out there.
Marika in Maryland