Asking for Hand in marriage, who to ask?

My girlfriend's parents have been divorced since she was 2. She has a decent relationship with her dad, and a really good relation ship with her mom. She lived with her mom and visited dad until highschool. The question is who do I ask for her hand in marriage. I had thought of getting all of our parents and close family together for a dinner to ask but i'm not sure if that would be appropriate. The dinner would include, my mom and dad, her mom and mom's bf, her dad and stepmom, her grandparents(very close to them too). Would this be an acceptable route?