Right, well like i said, just start conversation about her and maybe drop some real suttle hints about breast cancer awareness month and tell him your really interested in donating money to that charity, just be really nice about it. Its really hard to talk about someone who's passed but you can't avoid it completely. good luck.
You can stop for a while, meaby someone else knows about what type of cancer your grandma died of. Your dad was hurt by this so it's still a delicate subject. Meaby you can tell your dad is that you want to know this because you want to make a diffrence and help meaby then he will be intrested and meaby open up.
Aw thats too bad, how long has it been since she passed? if it's recent, give it some time. Then You can casually bring up a conversation about your grandmother and start talking about all the good memories about her but don't straight out ask the question. He'll tell you eventually.
she died before me and my cousin was born and before my aunt could tell my grandma she was pregnant in like 1990 so its been a while but i want to ask him because its breast cancer awearness month but like you said i dont want to straight out ask him.
Many cancers run in familys. Let him know you want to know which kind so that you can be proactive.