I live in a very small flat with my dp our 2 yr old dd and dss who is 17. He lives with us because bm's dp doesn't like him and bm has totally rejected him.
She won't let him into the house to see his sisters and various other disgusting behaviour. He is generally a well behaved polite, intelligent young man, any (normal) mother would be proud of him. She treats him like shit and always has done. On mothers day last year he went around to the house to give her a card and a present and she wouldn't allow him (and his cousin, same age) into the house because her dp was there, she told him to wait in the garden, which he did, for an hour, in the snow!!
Anyway the long and the short of it is that he is so confused that when he rang her today she invited him to go into town for a coffe with her and his sisters, he was thrilled. FFS she has broken his spirit down so much that he is delighted to be 'allowed' to go for a cup of coffee with her.
Her motivation is that she can tell her mum (her whole family is critical of her behaviour to dss) that everything is ok. She pretends to her work mates that he still lives with her ffs even though she threw him out 2.5 yrs ago.
Anyway my position has always been to try to remain neutral and be supportive and there when he needs me (hard to remain neutral when a teenage boy is sobbing in your arms cause his mum wont talk to him). I know that this is the correct behaviour and I do my best to just be a friend to him.
So today he rings up his sister and begs her to ask his mum is it ok if he calls around with a card. and I got really angry because his tone is so pathetic, it's as if he has done something wrong ffs. So I gave him a bit of a lecture about having a bit of backbone and self respect and not allowing himself to be put in a position to be hurt again. So she says he is allowed to go into town with them, hurrah!
His dad steps in and is warning him to be careful and realise that this is because she doesn't want him in the house and dss starts going on about how she is 'really trying to make things better'. FFS 
So a red mist descended I'm afraid to say and I lost it. And I said horrible things about his mum (all true though).
Basically things are really tough for us, money wise and space wise and health wise and dss has started to behave badly and do REALLY badly in school and is generally a problem teen. I spend a huge amount of emotional time and energy on him and I'm getting sick of it. He is lazy and rude and completely different than when he first moved in with us. He causes a huge amount of stress and strife in this house and I just felt like today was supposed to be a day (or morning at least) when I get a bit of very rare pampering and attention and what happens? We all spend the morning arguing about bm.
I can't believe that I am feeling so angry with him and I am very worried what the implications are on my relationship with dp are.
Sorry about the incoherent rant 