How can I ensure my trip to Disneyland?

I totally convinced my dad to take me to disneyland and we should be going in a few weeks! Except stepmom's vacillating now and making my dad really angry and not wanting to go. I hate when she does that cuz that's why we don't have a pool still. And everytime we start planning something right after she says she wants to go, she always complains and then says she doesn't. That's why it'll only be me n my dad and friend at the bball game tomorrow too. She said she wanted to go, and now she doesn't. Don't you just love vacillators? My uestion is, how do I make my dad not so pissed off at my stepmom cuz I havent been to Disney since I was 5. I really want to go and don't want to miss out. How do I do that and ensure my trip to disney?