What should the reaction be (a stepmom dilemma)?

I've posted questions before, and here I go again; my husband and his ex have a 11 y/o son; we live six homes away. we moved closer to be more involved in the child's life (mom got custody 4 yrs. ago after her older son ually abused the younger)...we found out from school and case search websites that the older son (age 19) who was in a residential home for the abuse was kicked out after committing ault, theft, malicious destruction of property, etc....his mom bailed him out and has him living in the house with her and the younger child (my stepson). we asked the child had he seen his brother lately (the teacher emailed us and asked was this a good thing the brother living at home) and denied it all until we told him we saw the brother in the neighborhood, then he admitted he was living in the house. We considered going to court to file an ex parte and remove my stepson from the house (we have the typed court transcript in which mom herself said the two couldn't live together), but didn't because when we went two weeks ago, for another reason to remove the child, the judge said mental abuse and neglect was not enough, it had to b physical (mom thru the 11 y/o out without coat etc. telling my husband the boy was our problem-- the child was sick and needed to go to the doctor's, and when he missed the school bus, mom called us and told us we had to pick him up).....well, according to the court decree, we have him noon xmas eve until noon xmas day, and then he go to mom-- the child threw a meltdown because "mom's having a xmas party and told me i could come home for it!! we tried to explain the decree and how she was telling him these things to upset him, and he asked "why would my mom do that?' we got him hm, and decided to open gifts that night (after mom came to our house, adn when i asked her to leave, her cussed me out in front of my husband and the child), an hour later, he asked could he call his mom, we said no,that this time was supposed to be our time. the child said mom won't be home at 12 b/c of work, and we told him he'd have to stay with us until mom got hm (whch would have been 2:30 ish). my husband left our house to pick up something he left at work, at 1, and when i looked outside and called for the child he was gone, i called my husband thinkning maybe the child went with dad, but he didn't. i went to the mother's house (knowing that was the only place he could go), and knocked on the door, but no one would open the door even though i heard voices telling the dog to calm down. my hubsnad seems to have given up on the child (mom kept the child from us the entire summer,and when we went to court, after my hsuband and i testifiying, she requested a postponment-that was in september, and nothing has happend since), the child gets frequent midquarter reports that works not done, adn we end up forcing him to do it, the child's unhealthy, mom won't take the child to counsleing, etc. but him running away on christmas was the straw that broke the camle's back.....i'm about to pack away his xmas gifts in the closet, feeling that if he wanted to run away from us, he's running away from all the fun things......we've called CPS, with never an answer, adn now witht he abusinve brother in the house, we don't know what to do......should i give up on the kid, or continue being the b**** (as he views me and his mother has called me in his face), knowing that will probably isolate him even more? realistic answers please