Small background, been with DH for 6 years and married for 4 years, I have a DD aged 12, DH has DS aged 11 and we both have a DD aged 3 together. DSS stays with us 2 nights one week and then 4 the next, on the nights he's at his mums we pick him up from school and he stays till she gets there and then she drops him off at ours again the next day for me to take him to school, on the days I don't work I pick him up and on the days i do work DH picks him up from after school club.
DSS has always been quite difficult, he's spoilt, selfish, arrogant and very immature. At first his behaviour was put down to him adjusting to my DD and me being there (although no concession was ever made for the upheaval my DD had had) but then talking to in-laws they say DSS has always been naughty and SIL had spoken to DH long before I was around about the possibility of DSS having a behavioural problem. DH wont have it tho, he says DSS just has a 'special personality' 
Over the years I have tried to detach myself and think that whether its a genuine problem (a teacher has suggested mild Aspergers once but again DH and his ex dismissed it) or whether DSS's behaviour is a reflection of poor parenting, either way it's not DSS's fault and I have loved and cared for him regardless.
Trouble is, I have a 12yr old DD and hell will freeze over before I allow her to behave the way DSS does. I try to explain to her that she is my responsibility yet DSS is not and I have a duty to bring her up in the way i feel is right but she doesn't understand why DSS gets away with everything and she doesnt. DD1 is never allowed to do anything without DSS so if say my sisters have arranged an outing on a day DSS is with his mother and asked for DD1 to go too, DH kicks off that DSS is being left out (assumes it's deliberate, which it is not) and DD1 isn't allowed to go. On the occassions I've put my foot down and insisted she goes, DSS finds out and all hell breaks loose. There is so much anger and resentment within our family its making me feel ill.
DSS is really mean to DD2, she's severely vision impaired and has a lot of attention with medications, hospital appointments etc so I guess I can see that perhaps DSS is jealous but he pushes her over and I've caught him deliberately sticking his foot infront of her to trip her up (which I think is even more cruel given she can't bloody see), he's hit her, shouts at her. DD2 got a new pair of glasses a couple of months ago but they went missing within a few days, I thought I was cracking up because I had been certain i knew where they were. 6 weeks later DSS's mum brought them in and said she had found them in a shoe at her house. Of course DH and ex thought this was funny, a mystery 
I didn't say anything for fear of yet another stop picking on my son he's an angel argument but DSS doesn't share clothes/shoes between houses, he goes in a pair of shoes and comes back in a pair, the glasses could not have been put in one of his shoes and then said shoes taken to his mothers, he wears the bloody shoes he takes. DSS has obviously taken them on purpose and then tried to cover it up. Why would he take his sister's glasses?
He's stolen money and jewelry from us and his mum yet he's never punished because 'we can't prove it was him' even when he has been the only person in the house at the time stuff has gone missing and the items have been found on him or in his room!!! He's stolen from shops and been allowed to keep the stolen item. He's stolen from someone at school also and his mum said it was girls jewelry and therefore he should give it to my DD1, why the hell did she not tell him to give it back??? On that occassion I took it back to school and told his teacher that he had stolen it, DH was very angry and said I should've said DSS had found it not stole it.
Overall I get on quite well with DSS's mum and she stays for coffee and a chat at drop off/pick up. She's such a nice person, she's reasonable and a responsible adult so why does she allow her son to speak to her like crap, to speak to me like crap infront of her? His behaviour is appalling and she says nothing, he punches her and the worst he will get is 'oh don't do that mate'. I mentioned a couple of weeks ago at drop off that DD2 had wet the bed the night before and that I was tired having been up during the night. DSS had overheard the conversation and dropped to the floor, grabbed DD2 by her arms and starting screaming in her face that she was naughty and she is not to wet the bed, she's dirty etc. I expected DSS's mum to step in but she didn't, she just watched as I whipped DD2 away from him.
Anyway, DSS's new tactic is to tell tales to his dad about me. We had an almighty row a few weeks back because DSS had told DH that I'm bullying him. I do NOT bully him! I told DH that if he is going to believe lies and that I am capable of bullying his son then I would not be taking his son to school/picking him up and he is not to leave his son in my care.
Nothing has changed since. DSS said on Tuesday, which is my day off, 'I'm at a club tonight, you will have to pick me up at 4.30'. I went to pick him up and got there at 4.20, once again DSS had got the time wrong and I sat in the red hot car till 5.10 waiting for him. when he came out of school he ran straight into the road without slowing down or looking so when he got in the car I told him off for it and also said that i would not be picking him up from a club again unless i have a letter stating where from and at what time as often I am running around schools in the area trying to find him and usually the time is wrong also. DSS told DH that I had called him an f'ing idiot, a cn etc etc. DH believed him and yet another row ensued, DSS later admitted that he had lied but no apology and no punishment for lying.
DH and ex have now decided that DSS is responsible enough to have a key to our house and to walk the 3 miles home by himself, they've cancelled after school club and from today this is the arrangement. DSS can't even cross a road properly! I'm not keen on him having free reign of my home without supervision what with the stealing etc and I honestly dont trust him, he often does really stupid things and hurts himself. I know he will only be in the house alone for an hour max but still I just dont think it's right. I wasn't consulted and even if I had been I doubt I would've said anything for fear of yet another argument.
DH and I split up for a few months last year, for many reasons but one of them being DSS. I wish to god I had never gone back 
Sorry for the long rant, if you got to the end then thank you for reading.