My Ds3 didn't sit till 8 months, He did roll once at 7 months and then never did it again till he was 12 months.

He did start crawling at 9.5 months and was walking at 12 months..

Jacob had teeth though but was a great sleeper. and he was a big boy too.

Personally I think Jacob was just so content as his older siblings got him everything he pointed to. As I said above he was a big boy so I think that is why he also had trouble rolling and he hated tummy time