It is not uncommon for women to develop sz during their late 30's. I was diagnosed at 37, but I had had depression and mild symptoms of sz for years. Up until that time, it just had not been severe enough to impact my ability to work or sustain a relationship. Then suddenly my mental health was severely compromised and the sz was so serious I had to be hospitalized over and over again. I think I could have avoided some of that if I had taken the prescribed medications. But, as sick as I was, I would take the meds just enough to feel better, then I assumed I didn't need them anymore. Wrong! Encourage your sister to find a good psychiatrist and to be committed to a treatment plan. You might also tell her about this web site -- the people are great here.
Carolyn