How to meet fathers' new girfriend?

After dealing with about a decade of alcoholism incident with my mother, my father finally filed for divorce this past November. My mother went back to rehab, and has finally cleaned up her act for the time being- she has been clean for three months, and is back at work at my fathers office, working as a secretary. About 40-ish years ago, before he dated my mother my father dated a women we'll call Diane. He and Diane recently got back in contact before he filed for divorce, Diane has recently filed for divorce and moved from her home state to FL, the state in while my parents currently live to help support her parents, who retired down to FL. She and my father started dating again while my mother went to rehab, and my mother moved to a halfway house in February. I recently got a call from my mother saying that the halfway house she was staying in put all her belonging she had with her in storage and dropped her out because she had finished her program, but as her bills had not been paid the house would hold her stuff until the bill was paid. I called my father, whom I had recently loaned a large amount of money to, only to find he was at a show with Diane, while my mothers belongings were locked up and she was staying with my cousin while he was out having a good time instead of paying the bills. My father wound up paying the bills a few days later, but 2 days ago I got a call from my mother saying that my father bought Diane a set of golf clubs and had them sent to the office; not Diane's home, where it would have been more appropriate, but to the office in which my mother works. She had to sign for the package. My sister still lives at home; I am in the Navy and stationed in Virginia, so I don't get home much- all information I get is mostly second hand.My sister is going to college supported by my father.; he pays her car payments and she goes to school. Today my sister called me saying our father had sort of kicked her out. He was late on a car payment (I still have not gotten back the money I loaned him), so she asked about it. He'd had a bad day at work, understandably, but when she brought up the fact that instead of paying the car payment on time (not the first time he's been late) and bought Diane a set of golf clubs, and spent $400 dollars on tickets to yet another show, instead of paying the bills, my father blew up. He told her to pack her things and find a job, a way to pay for school and a place to stay because he was going to put the house up for sale. My mother is currently staying with a cousin because our father has brought Diane into the house to stay overnight from time to time, maybe once a week; my mothers things are still in the closet, her makeup is in the bathroom, and she still works with my father. She does not want to encounter Diane, and I do not blame her. My sister was supposed to take a trip this weekend with her girlfriend and my father and Diane to Flagler. My sister expressed that she was uncomfortable, Diane must have sensed this, because she (Diane) told my father that my sister seemed uncomfortable, and they called off the trip. Since Diane clearly cant win over my sister that easily, I am afraid she may turn to me when I travel down there next week for Pover. Is there a way to be diplomatic and polite about this without arousing my fathers ire? I am not comfortable with the situation, and I have not been home to see this first hand.