Questions: Family & Friends

I am just trying to understand why my mom continues to let my brother and sister in law live with her and my step dad and take advantage of them. They showed up to my moms broke and up to their eyeballs in debt almost a year ago and today they are no closer to moving out then they were from the day they arrived. I can see that they have no realistic plan to do so either. My mom has yet to see even a penny from them as far as help with any of the bills around the house go. On top of that they don't help clean up around the house or do anything to help out. If you try and tell them anything about it they get offended and get defensive and ugly. My sister in law in particular is very disrespectful and just whines and complains about everything and doesn't appreciate a damn thing. My brother just kind of goes along with what she wants to do and pretty much plays the victim mentallity that everything in his life is a result of just bad luck and not his bad decisions. It doesn't help either that my brother now owes about a years worth of back child support. When I try and tell my mom that they are taking advantage of her and that they need to start helping her out she like to bring up that they don't have any money. That's bullshit because they have money for pot, money for beer, money to go to concerts, money for water parks and having fun and being cool, but no money to help her out with the bills around the house. So to put it in short they are both very selfish and ungrateful individuals. I tell my mom that she needs to just kick them out. If they don't want to help themselves then why is she bending over backwards to help them. I can imagine that my neice, my moms grand daughter, their daughter has a lot to do with it. I don't see anything changing though. I tell my mom they will figure something out if she kicks them out. Why won't she just do it, and why does she continue to put up with their shit?