Well, just about all 2 month olds are weak. My daughter is 8 months, and I have the funniest pictures of her doing tummy time. She haaaaaaated it and would just end up eating the carpet in defeat. I don't think its odd for babies to hate tummy time or to vary from sibling to sibling regarding motor skills. I think the most you can do is ask your ped if she seems to have low tone, but they really do develop at their own pace. For all of the defeatist attitude my daughter showed when she was first born, she's on fire now, developing by leaps and bounds. She still can't crawl but she slides around all over the place. I can't remember when she turned a corner with tummy time, maybe at 4 or 5 months. Just try to relax and enjoy your little girl and don't compare to her brother. I. Know that. Must be hard with twins, but they are going to develop at their own pace.