I have S/Kids, SDD 15 and SDS 17. I have known them since they were 7 and 9. They were delighted for me to marry their dad, but dad - my DH - wouldnt let me discipline them, or be what I call a normal step-mum. By that I mean, I couldnt call them from the TV if dinner was on the table, or ask them to take there weekend bags to their room, or take their plates to the dishwasher, or ask for help preparing the dinner etc. He was quite happy for us all to go out for family days out, and for me treat them, molly coddle them etc.
I totally understand that he felt very guilty over the marriage breakdown and that he just wanted to please his kids and not upset them with this kind of stuff on their visits, but it was and still is extreme 'blcokage' behaviour on his part. To me, helping with family chores like this is no hardship - and what I have mentioned above is it. No more. I dont expect much from them, but cant even epect the 'not much' to be met.
It has caused rows - yes. But for the sake of my relationship with the kids, I learnt to keep my mouth shut over the years. I now have a good relationship with them, and in answer to your question, yes, I could contact them direct if I needed to. I usually do this to find out when they will next visit, or to remind them about sending dad a bday card etc.
Its not easy being a step-parent, as you dont ever want to replace the real parent, but for my part - I count friendship with them more important than being a parent.
Hope this is all making sense, and helpful.