It was nice of your mom to take him in, but if she's allowing your sisters to act like this, that almost erases her good deed! You didn't say anything about what you've said to her, but even if they only act this way while she's not around, it's her job to know what goes on in her home. So make sure she's aware of all of this.
You can't change how your sisters treat him, because you have no power over them and they know it. But mom has loads of power to get them to shape up. In fact, with the 18 year old, mom isn't even required to let her live there any more. It's now legal for mom to kick her out. Maybe mom needs to issue that as a reminder.
Meanwhile, keep being the good friend you've been to him, and feel free to rip your sisters a new one if they say anything while you're around. This is astoundingly cruel. But mom is the one who needs to fix it.