Well, my kids are 16 and almost 10 so not quite where you are but I can offer my opinion. I think if he is 20, you can lay down some rules. Either out, or show some respect. At 20...come on, he can get a job and get out. I am sure it is easier said than done, but I do tell my 16-year-old he is not staying at my house for free while he does nothing. So after he graduates, either college or work, but he can't disrespect me. Don't get me wrong, my son is a great kid and I love him a lot but he has his moments of testing his limits and I am very firm that I will not (and do not have to!) tolerate rudeness and disrespect. I can imagine it is a million times harder with a stepson but you have been there most of his life! Best thing is stay calm. Whether cantankerous stbx or kids, they all like to get a rise out of us and the calmer we stay, the calmer they get. When my son gets nasty with me, I tell him I will get a recorder so he can hear how he sounds. Can you do this? Just calmly tell him how he sounds and say you will record him. Then do. You can either get him without him knowing, or give him the choice to willingly be on film acting like a 2-year-old or get on his better behavior for camera's sake. You are right, your mentally disabled daughter needs and deserves much more attention and care, as well as your youngest. He is an adult, about time he starts acting like one. Do you think he's put out by his half-sisters? Maybe that's why he acts up. No offense to him but I think he should be more mature. I don't know. I'd just focus my attention on my daughters. Let him come and go. He should go soon enough and live his own life. The more you argue over him the more he wins and I can imagine the more he smiles. He probably needs his dad to work through stuff. Your husband should step up. But you can't force him to either. Really tough, but I'd seriously back off of him. I am sure you love him so just be there as best as you can, but when nastiness starts, put it to an end. Boys take a bit longer to mature, hang in there. God knows that's all we can do sometimes!