First thing...If you're this concerned about your brother, then you should definetly tell your dad the whole story. It sounds like your mom needs all the support she can get, so if her and your dad can work together then I think that would really help a lot.
Second thing...Get linux on your mom's computer
Third thing...If he tries to sneak off to play games at his friend's house, then make strict curfew times. That way he's only playing a couple hours, instead of half the day hehe. Plus at least he has a tad bit of socialization if hes at a friends house. If he breaks curfew, then ground him for a couple days or whatever. That way he's in the house, doesn't know how to play games on your linux systems, and now has to figure out what else he can do with his life.
Fourth thing...Get him to start doing other activities. Whether its playing a sport with you, joining a club, doing community service...just something else besides computer. He probably won't do it alone, so you're gonna have to do it with him. Heck, even take him to the movies and try to get him to socialize with other kids his age. It may be a little embarassing for you, but if you want to see him change then you and your family has to help him. Also, don't show that you're embarrassed or that you really don't want to do this. If you show that, then he's just gonna feel like a burden and think he's better off just staying at home.
Hope this advice helps you. But yeah, the first thing and most important thing is to let your dad know exactly whats going on. Its important that he knows the truth before he lets your brother move in. Best of luck!