Hmm, biasanya 8 bulan dah boleh sit up dengan minimal support, tapi memang ada babies yang focus on crawling dulu before sitting independently, a few of my mum friends punya babies macam tu jugak. Since dia dah boleh merangkak well, yang penting dia ada muscle control, tapi I'd suggest check with your pediatrician just to be sure, especially to rule out any muscle tone issues lah.