Sorry I have read your post twice and am still not sure I have this right?
You have 2 teenage stepchildren and 3 (2DD and 1DS) with your DH.
I'm assuming the ages at the bottom of your post are for your children?
How old are the step children?
Ok - that's it for the questions 
My first thought is that it's inevitable that the "family" dynamic changes when there are additional people in the house.
My DSD used to spend pretty much half her time with us, but as she became a teen that has lessened simply due to her social life. She wants to be with her friends at the weekend who invariably live near where her mum does.
I've noticed that since her "visits" have become less frequent there has been a change. Recently it's more like she's a guest in the house rather than being (as before) her home.
It does throw things out of balance. She seems to expect to do what she wants as she's doing us a favour by being here (what to watch on tv, what to eat, where we go etc all of which DH indulges) rather than simply integrating with her half brother, me and DH as she used to.
My DS responds by playing up (and getting told off by DH which has resulted in him thinking DH loves DSD more than him) and the end result is tensions run high all round - apart from DSD who seems totally oblivious as long as DH is letting her do whatever she wants.
Please don't get me wrong - I love her. It is however very hard and like you I have got to the point that when DH tells me she's not coming this weekend, unlike a few years ago when I would have been sad, I now confesses (with huge guilt) that I feel relieved.
I used to think I was a good SM but now I'm not so sure. Obviously I hide these feelings but the truth is I feel more and more resentful of the impact her presence has on family life and especially DS. I've tried to speak to DH about it, but he understandably wants to encourage her to spend time with us.
I live with it for now as given her age we have 3 years before Uni and the situation will change again.
I'm not sure how helpful that was - certainly no insights, but you're not alone in your feelings.