Aunt and grt-grandma (dad's side) died of ovarian cancer. both post-menopause (aunt 59, ggm late 60s), both had kids in 20s. heredity or coincidence?

Ovarian cancer: Hereditary ovarian cancer is a distinct possibility with brca 2 or 1 being the most likely choices.

Likely coincidence: Hard to say, but the association is much stronger with first degree relatives (sisters, mothers, etc). Your history does not rule out a hereditary component, so make sure you let your primary care physician know.