Hi L.:
I'm a pediatric occupational therapist, and although I don't work with babies I know about development.
The new trend of sleeping on babies' back is cheating babies out of the important tummy time. Tummy time helps develop upper body and arm and head/neck strength and control! It's really crucial to development! Leads to crawling and creeping, too, also important for development.
At 6 months your baby girl is in the middle of the timeline for developing rolling and she's fine to be sitting now. So, don't worry for a few months re. rolling. I reread & read others ideas...it is true babies hit milestones at their own time. So, if other development is ok, you'll know when to really "worry" about it.
BUT, encourage her to enjoy tummy time! Get down there on your tummy, too, and entice her with facial expression, singing, toys she likes. Just keep up with it so she begins to enjoy it.
Rolling tummy to back kicks in between 4 - 6 mos. And back to tummy between 5 - 7 months. So take it easy and get you & your partner & her invested in very interactive fun tummy time. That can then lead to pushing up on her arms, extending her head & neck against gravity, developing her chest and tummy & back muscles, too.
Have fun, good luck, Jennifer H