you can ignore this is you want, but wanted to offer my view...
i was the stepchild/teen. my family set up's a little different, but trust me, it's an "oddball" relationship from either side
i don't know how the kids act towards you or how often they see you, but it was the little things that made me smile (corny but true). she would always have something in my room for me (magazine, soap, *something* that i knew was from her) but she never pried or imposed herself on my time with dad.
in talking to my step-mom now, i know she was really worried about when to speak up, when to join in, and all i can tell her is that i wish she'd done more. so, i say, feel it out. you deserve to be there, it's your home, too. the more they see you as part of the group at their "dad's house," the easier it will be to form personal relationships with them.
obviously i have no idea nor can i imagine how it feels to be on your side, but i wanted to add input. ignore at will