Wow, your son sure was crawling, cruising and walking early. Your baby should be able to roll from tummy to back by about 3 months of age. By 5-6 months old, she should learn to roll from back to tummy. 6-7 months she should be sitting up. Yes, all babies develop at different rates. She may skip rolling over but be able to sit up. That tells you she has the strength, but not the desire. However, you still need to let your pediatrician know if your child isn't meeting "normal" milestones.
I have 4 kids. Two of my kids hated tummy time and didn't meet "normal" milestones for rolling over, sitting, etc. As they got older, now 2 1/2 and 3, their delays became more pronounced. At 3 years old my son had the motor skills of a 13 month old. Both kids receive occupational therapy to help them catch up and increase their muscle coordination. I don't say this to scare you but to let you know that muscle strength is important and can be helped. There is nothing "wrong" with my boys, they just need some extra help to develop that part of their little brains and bodies. I encourage you to talk to your pediatrician. Best Wishes!