How to tell my mom i got molested by his father

So this happened years ago and I'm still keeping it from my mom. I'm scared to tell her the truth, my grandpa looked so nice and no one would expect him to do it. He molested me multiple times, I told my aunt (Ann) about it and cried infront of her but all she said is "did he really do that" then proceed to talk about another topic while I'm crying.

I'm not the only one who got molested by my grandpa, My aunt (uncle's wife) got molested too but my aunt (Ann) said "it's her fault for wearing shorts, she knew my father is old"