I guess I can ask here, since I know my mom and step-dad couldnt find it... But here's the problem.

My mom has this past of somehow dating abusive men. Her boyfriend, when I was only six or seven, had abused her and me. I dont want to get into the details. *sigh*

When I threatened to call the police with the huge bruise on my face, she finally got the courage and after. That man who she dated still lingers in my nightmares.

So now, she is dating someone knew. Someone who, like in the past, she constantly fights with, seems to have little to nothing in common with him, and seems to unhappy about. she complains about him all the time. He's hit me, yelled at me til I cried and punished me for no reason or if I did the smallest thing wrong. *shrugs slightly* Now, they have a baby, my sister who I love with all my heart. I am scared shitless for her, because I am going to college in only a few months. She wont have someone there for her, because my mom never sticks up for me, even. I'm scared he'll hurt her, and I wont be there to help her and call the fucking cops.

What do I do? I want to be there for her but the most I'll get to see her is during x-mas/thanksgiving. I'll be in my dorm almost 300 miles away for the rest of the year. She's only a year and a half. *sigh* So I dont know what to do to keep her safe.

I haven't really got any advice (except to call Social Services) I just feel bad for you, that's a terrible situation to be put in

Yeah , that would be the safest thing to do. Or call childline if you live in the UK. Social Services will be able to sort that kinda stuff out. Or if this man is abusing your mom you can go to the police station. Thats the only advice I have

The problem with this is I have no proof...

It makes me feel terrible that others have to go through things like this. Where do you live? As Abstract said, call someone who you can talk to.
I don't know what else you can do if you have no evidence, but whatever don't be so stupid as to lure him into a situation where you might be harmed just to get some evidence.
I know much of the time social services are damned useless but it might be worth informing them anyhow.
If the baby is his then he might treat her differently, many parents do treat their own differently to those of their partner.
Try talking to your mum, you may think she doesn't listen, but you may be confirming her own fears.

I sugest to call someone maybe a relitive or if your despert call social services,or just call the cops but i guess that wont do good if you have no proof.

talk to a realtive if u feel comfortable while your away and maybe they can come over to act like there excited to see the child and make sure the child doesnt get hurt.