Just thought I'd come and update my thread. DS and DSC have only spent 1 day together so far and the atmosphere has been very peaceful, which is nice. However, this is because 1. when we have DS alone, things are understandably easier than when we have 5 kids and 2. DP is letting his kids do what they want, when they want and no chores at all, so it's not surpising they are being pretty well behaved.
Of course, when DS gets back from his Dad's, I daresay DSC will tell him all about how they didn't have to do anything while he was away, DS will then get into a strop when he is expected to do chores and DP will tell me how much nicer it was when DS wasn't here.
DS will be expected to write thank you letters, DSC don't have to nor do they even remember who bought them what. I think that is terrible bad manners.
When we did have a day together, we went out for a meal and DS and DP end up arguing about something non consequential but DP has to win. Whilst I spend my time asking DSD4 not to snatch, not to interrupt and to remember her manners whilst also dealing with DSS3 who takes DP's phone and hides it, pokes DP in the face, kicks DP and of course, nothing much is said. DP ended up telling DS that he won't bring him out again, because DS dared to argue with him about which dish was spicier! From that point on DS was understandably a bit peeved.
I am going out and staying with a friend tonight with DS and I am soooo looking forward to it.
Had a bit of an issue with DSS and money. Turns out DSS has been taking money from here to his Mums house, I am also certain that he has taken some of DS things there too without asking and all of DSC will take DS things without asking and not return them. It is only me who stops this.
DP's answer to his sons stealing problem, to hide his money away and tell DSS he has done so. No talking to him, no finding out why, no punishment, no consequences.
I was having a panic about DS xmas money going missing and DS was at his Dad's. I asked him where it was as I was frightened it had gone missing. DS had hidden it right away. This used to be the boy who could leave anything anywhere and know it would be safe. 
Oh and our cat died - feel very sad that she was "our" pet and that I have lost something else that was just "ours" rather than "everyones" (which means step children only and we don't get a look in!) DS took it quite well, but even DP snapped at him then, when he demanded that DS give me a hug when I was upset. Funny, how he doesn't normally like DS showing me affection. DSS aged 11 showed a lot of sympathy by laughing and poking the dead cat.
Despite all this I don't feel too bad, I suppose I have just been very busy with 4/5 kids and lots of Xmas entertaining etc.
I'm very nervous of spending time with DS and DSC and DP as the atmosphere is tense and I feel on edge all the time. Something has to change and only DP can do it.