my mom basically adopted a lost boy

So I'm 20 and moving out within a couple
Months anyways, but my 18 yr old brother has this friend that I guess doesn't have a place to stay. I don't know his back story, I haven't asked. He's 16. My brother has friends over all the time, but this one hasn't left our house for a month. I asked him if he lived here like I don't really talk to him but I still asked him and he said "kinda" 
And I was like ooookay lol 
My mom is also moving, and he has packed all his stuff too. He's moving right along with my family. I asked my mom and she said she didn't plan on him to stay that long but with me moving out she'll have room for him. "He's not going anywhere any time soon" She's making him get a job so he can get a car and all that. He's basically like a part of our family now like my mom just took in some random teenage boy out of nowhere. I know that she talks to his dad and his dad cares about where he is and wants to know what's going on in his life because I've heard her on the phone with him before. I don't even know??? I think his dad might have gotten remarried and he hates his step mom with a burning passion because she treats him badly. This is just gathered little bits of information and I feel weird asking him because he's really quiet and standoffish, but he's well behaved and respects my mom so whatever lol  WHY do you bitches always assume there's a problem. It's not a problem, I just didn't expect my mom to do something like that.