How can my boyfriend and I convince his father to let his younger sister move in with us?

Alright, well my boyfriend Joey is 25 (I’m 23) and he’s absolutely amazing. We’ve been together for almost 3 years now (We live together as well) and I love him more than anything. Well, Joey has a younger sister, Kim, who’s seventeen. She’s a nice girl, and I know her pretty well, because Joey thought it would be a good idea for me to spend some time with her because they lost their mother about two years ago, and their father is hardly around (sort of an absentee father, Joey really resents him for it; he can’t stand his father. Joey says he got so much verbal and emotional abuse from his father, and Kim has been having the same problem. Recently, he’s started drinking a lot, too, which really concerns Joey because his sister is living in that house. Kim says he comes home at 3 am all the time and he’s usually drunk and gives her a hard time). So I’ve sort of taken her under my wing and she calls me for advice, and sometimes we’ll go out to eat or go shopping, etc. Well a while ago, she told me at lunch that she’s pregnant. I tried to help her out, and I was there when she told Joey. He was extremely angry at first; he was furious with her, but he’s calmed down and he let her know that he’ll be there for her the whole time, and I know she was really relieved, since Joey is pretty much the most important person in her life. He’s an amazing brother to her.

Well, Thursday night, Kim told their father that she’s pregnant. He had a fit, and told her to get out of his house. She called Joey at 1 am and he went and picked her up, and had a very… Strongly worded discussion with his father. Kim has been staying with us since then, and she’s really worried about where she’s going to go. Yesterday, Joey and I discussed it and decided that she could live with us. We’re both fine with this, and Joey wants to do anything he can to help her. However, Joey went to talk to his father about Kim moving in with us, yesterday, and his father had a fit, telling Joey to stop putting himself in the middle and trying to make his father “look like the bad guy.” Now Joey doesn’t know what to do because his father is threatening legal action, and he’s drinking heavily. I know Joey is very worried about Kim, as am I. What should we do? Joey is extremely stressed out; what can I do to help him?