Tiff,
Don't stress too much about those 'developmental' milestones. I had the same thing that one twin was always behind. he was my sickkie as well. Their poor bodies just can't grow, get better as well as reach milestones all at the same time. I found that with time my sickkie caught up once his health issues were more under control. I thought that we were going good, and after every MCHC nurse visit, or doctors visit, they would find some other medical thing that we had to worry about, and put us back to square one.
In hindsight (such a wonderful thing) he was doing great, and so were we. It just that those damm charts etc that they compare all babies too were getting in the way. Those milestones that they talk of are for full term, singlton, well, children. Not prem, twin, and sick kiddos. You are doing a fantastic job with your twinnies and don't you ever doubt yourself !!!! Who cares if they cant grasp a toy when other babies are suppose to - she will get there when she is good and ready!
My twinnies are now 3 1/2 and I was still paranoid that he wasn't meeting milestones as the docs were doubting him, even though I knew that he wasn't. So I got him tested by a psych at the RCH. Guess what!!! He is exactly reaching the milestones for his age. So screw you docs who doubted that he was, and screw you docs that doubted me and my maternal instict.

As long as she is happy and secure- the rest will come with time