I should feel pleased, right?
I have lots of mixed feelings, grateful that they're not fighting all the time, but not convinced the relationship is as healthy as it could be. They're both 14. Some of this is probably my own insecurities too, I will admit.
DSD literally follows DS around the house, sits in his room when he's not there, asks constantly after him if he's out, is giving up clubs, friends and not seeing her boyfriend and choosing to spend time watching silly you tube videos on the computer with DS.
I expected DS to be moving forward with his life at this age, of course, and instead of him spending more time with friends and less with me, more time at clubs, more time on homework etc. etc., he's spending it sitting with DSD, and not seeing his friends. He's "hard work" when she is around (which is always) and the fun times and closeness that DS and I have always had feel like they are ebbing away.
He tells me that he can tell her anything, but then admits she never talks to him about anything about herself or talks about her feelings or her life. She just walked out of her Mums a few months ago and has barely been back since, no real explanation at all.
I have to almost force him to spend time with me, and of course, DSD is there too. They literally don't spend a minute apart. When he chooses to have a bath, she will have a shower (not in the same bathroom!), when he does his homework, she will do hers, when he watches tv, she will sit with him. DS and I have always had a close relationship, and to be honest, I'm feeling resentful that DP children are calling the shots yet again, or that is how I feel. They always get what they want and in this case, it is 100% attention from him whilst she distances him from his Mum. Even DP has said that he thinks his children are jealous of my relationship with DS.
DSD is critical of DS and undermines him all the time and I am worried about the effect it is having on him. I'm sure she feels insecure since just deciding, in what seemed a split second, to move in with us. We almost have to force her to see her Mum and it's only a couple of hours at most in several weeks. She is always at home and her school hours are less than DS, so I can't even get the odd ten mins here and there. He never sits in her room, never seeks her out, but it's always her following him around a bit like a puppy. It's like she doesn't know her place at home. 
When I try and spend time with her (or DP does) and we ask how she feels about something, we just get a "meh" answer. She just wants to be with DS.
How does this sound to you step parents? Am I being silly?
Also bear in mind, that DP and I might not be continuing our relationship, and if so, not necessarily from the same house. How is that going to affect DS and DSD?