I had testicular cancer and his sister died of ovarian cancer. Is this just a terrible coincidence or could it be hereditary?

While these are both. Can be germ cell tumors, there is neither familial or molecular links to cancers of either kid, or the oddity of siblings of opposite sex acquiring them. Most ovarian tumors are not germ cell, but epithelial, the opposite is true of testicular cancer.
Likely coincidence. Sister's ovarian cancer would qualify her primary relatives (offspring, siblings) to have breast-ovarian cancer (brca) gene testing, especially if at a young age and if other family members had breast or ovarian cancer. Brca mutations can be associated with pancreatic or prostate cancers, but not testicular.