Honey I know it's hard on you but the best thing you can do for your sister is make her grow up and stand up for herself. Until she does that, she'll keep falling back with these types of guys. She has to leave and there are places she can go. She probably doesn't want to go to a woman's shelter, first of all they have you press charges against your abuser, she doesn't want to do that. You wrote how she gave her car up, I'm sure you kept telling her she'll needs it, but she did what she or her boyfriend wanted her to do. Until she wakes up and sees everything how it is, then she'll stay and be miserable. I'm betting he's hitting her even if she doesn't admit to it, if he's hanging up the phone, taking that much control, she's being hit too... If she does have a job, she can take her paycheck get on a bus and leave everything behind, it's just material. When she has had enough she'll do that. I know you want to help and protect her, but she's has to be an adult now and grow up.
You could find a shelter in your area, church group that may take her in. Tell her take her next check and show she's done with this guy. That she needs to start fresh and show she's ready for a change and you will help her. Don't give up your kids room, let your sister prove herself, not only to you, mostly to herself. It's tough love, but she needs the backbone to stand up and make the change.
It'll break your heart that you feel you aren't helping enough, but this is why you have to be strong for her. She can do it on her own. Don't give her money. If she moves close to you, buy her a meal, or dress for work, but do not give her money.
I'm praying for you!