In the middle

For the longest time I've waited, to be able to have a relationship with my youngest sister. For so many different reasons, it has never happened until now. My parents divorce was so nasty, that I didn't see my dad for almost 10 years, except for the most sporadic of occasions. Sometimes, I'd see him at Christmas, sometimes, I'd get a call for my birthday, stuff like that. And rarely, I'd be able to see my sister who is 10 years younger than me.
Now, with the wonder of the internet combined with the fact that she's finally 18 and can make her own decisions to see me, we've been able to keep in touch, get together, and start to do "sisterly things".. It's been great. I love getting to know her, and her getting to know my children.
In the last couple of days, my dad had been calling. Leaving messages for me to call him back. The last one it was an "emergency". So I called one night before heading to a soccer game. His emergency was "are you with me or against me?".. Now him and my step mom are divorced, and they are having custody/child support issues. I won't get in to them here, but do you see a pattern in just the little bit I've written?
They apparently go to court next week, and he has the balls to ask me if I'm with him or against him. I grew a pair, and told him, I'm not on anyone's side. I'm on MY side. I'm not getting in the middle of their issues, to make it yet another issue to not be able to have a relationship with my sister. Her and her mom are pretty close, so I'm assuming, if I was "with him".. I'd be "against them". It's crap. And I can't believe he had the gall to call me and ask me that. And to say it was an emergency!!! He said he wanted to know what he could and couldn't tell me. Like I'm going to run to the closest loudspeaker and shout to the world what he tells me.. I could care less about their issues. My only issue is that I'm getting to know my sister, and nobody is going to stop me. He didn't like what I had to say. He started yelling at me. Told me I hurt his feelings, and that he couldn't believe I wasn't "on his side."
It really bothers me, that he would even attempt to put me in this position. To make me choose. After him not being there for me for so many years, I'm supposed to put all that aside, and just be on his side? Are you kidding me?
Maybe there's a reason people start to abandon him. Maybe there's a reason he wasn't in my life. It's made me a stronger (yet somewhat emotionally issued) person. I'm able to see the whole picture instead of just the little one staring me in the face. How dare he? I just don't even know what to say.
I feel bad for him, and want to be there for him. But not if he's going to try and put me in the middle...