7 months and counting.

My little nephew turned 7 months 2 days ago. I took some pictures but haven't downloaded them yet so....say tuned! Anyway, we have a few concerns about the adorable little guy :( .
He is NO where near sitting up yet. My sister was a slow go-er as a baby too. She has cerebral palsey. Shannon didn't sit up until 10 months and was nearly 4 before she walked on her own. We are unsure as to if Zackery is suffering from low muscle tone like his Mama, or if she just isn't giving him enough opportunities to practice using his tummy muscles. It is likely the latter, or both. We (other family members and I) have noticed she still lays him on his back ALOT for play time. She doesn't like to hear our advice however. We are praying for wisdom to my sister, as well as a change of heart. We are also praying Zackery does not have low muscle tone, and that he catchesup quickly! He is bright eyed and very alert so, at this time we are not concerned about his mental capabilities. In fact he seems to be very bright. Sometime you can just see those things in their eyes!