Preemie twins don't seem to be developing as fast as they should.?

I have twin girls who are 7 months old. They were born almost 6 weeks early. Neither one can sit up yet. One is starving to roll herself around the room, but the other has shown no interest in moving about. She simply stiffens her legs out and twirls her feet. They are not on state insurance yet, its in the processing, so they have no doctor for me to take them to. However, I am concerned with the smaller ones development especially.