And he is leaving me to take care of my little sister, who is developmentally disabled.
Well...you may not be judging him, but I am. He is leaving you to care for HIS disabled daughter?!?!?!? That is just wrong. Just wrong. You deserve a chance to get your life started without the huge burden and responsiblity of caring for someone who can't care for themselves. I'm sorry your dad has on-going responsibilities for his daughter (and you), but that is his lot in life, and, from what you are saying here, he is abandoning his responsibilities. I just hope he comes to his senses before he does some real damage to you and your sister.