BM kicked SS12 out
Sunday my DH received a phone call that he needed to come pick up his son from BM's house. She's a bitch, but SS12 is a total a-hole. He's ADHD and talks back to EVERYTHING you ask him to do. I can't stand him. Honestly. Just thinking of him now makes my blood boil.
DH works a job where he's out of the house from 11am-11pm. I'm the one stuck with the brat. He didn't even ask if I would watch him. He just expects it. I understand that it was an effed up situation that brought him over, but my DD has told me before "i hate you" but I ddidn't hold it against her. Apparently, BM doesn't care to work things out.
It has been hell. Today is tuesday & SS finally went home. I've spent 2 days in my daughter's room (she's visiting her grandparents for a week before school starts since they live so far away), been told my DH has chosen SS over me, and have been told to move back home.
I'm wondering if me bringing up the fact he couldn't even ASK if I'd watch him was out of line. I mean, I'm the kids' stepmom afterall. I just can't believe it's blown up into a huge mess to the point where we're both talking about divorce. This is definitely the worst fight yet.
Feeling down. And sad. And just venting a little bit. 
I feel very sorry for you,
I feel very sorry for you, and can completely understand what you are going through, as I am currently in a very similar situation. I have SDs 16 and 14, and their BM is a nightmare. All three are very combative, unreasonable, argumentative. I am very quiet and hate confrontation. For the last couple of months BM has been threatening to throw out SD16 and telling her and us she has to come and live with us.
I could not live with her (we have her EOW and that is bad enough) and have told DH that I am prepared to move out if she comes here permanently.
It is totally not unreasonable of you to be angry because DH assumed you would look after SS without even asking you. A lot of SMs on the forum (including me) would never agree to look after a SKID in the absence of the bio parent. Quite aside from the fact that I disengaged from the SDs 8 years ago, I would never agree to that. You said "been told my DH has chosen SS over me" - who told you, DH himself or the SS? Assume it was your DH, if this is the case, the prognosis for the marriage does not sound terribly rosy. But he is being utterly unreasonable - console yourself with the fact you are not at fault for hating being landed 24/7 with an extremely difficult boy with a disorder which would try the patience of a saint.
My bm kicked out my ss13 last
My bm kicked out my ss13 last month. I refused to take him into my home. At all. He came for a visit and after 24 hours my dh made arrangments for him to go to my MIL's because of his behavior.
You have to set some boundaries. I know it may be tough-but you are not required to care for this child-especially since he is obviously so unpleasant. Perhaps suggest to your dh that he put more effort into parenting the kid and teaching him more appropriate behavior and you might be more willing to watch him. I spent years consistently putting the responsiblity for my ss back on dh.
He wanted to blame, demand, etc as well. I told him that essentially ALL of ss's behaviors were his responsibility as he is the parent and all tasks, chores, etc are all his responsibility. It is his job to do ALL the work. I told dh that if he needed my assistance at times, tha the MAY ask me nicely if I would help and if I was available then I MIGHT chose to do so. But I guaranteed him if he (dh) was going to be an asshole about things then he could just assume that the answer will always be NO.