How to explain these things to my 4 year old

Those of you know the situation with me walking in on my father molesting my 13 year old son in his sleep. Of guess what else my wonderful mother has done. She now made a go fund me to help pay for her and my dad's honeymoon after they renew their vows. I have never hates my mother so much in my fucking life.

Anyway my 4 year old daughter has had so many questions about the what's going on. (She was never touched by my dad. She always shared a room with me and she's talked to someone) She asked why my sons dad is with us(to support my son too). Why her brother is wearing diaper(he's wearing drynites for the bedwetting which I told her not to talk about to him) and the biggest question that I have absolutely no answer for... Why are grandma and grandpa not here? Where are they? When will we see them? I miss grandma and grandpa?...

Edit: She did use a gofundme before to bail him out but lied about the charges.