Hi c3lin3,
Your baby will have to devt assesments done either by Polyclinic or PD. You may like to check out with them about your concern and get professional advice on how to stimulate your baby to sit up. Usually, a baby should be able to sit unassisted by 8 mths (as a guide).
However, as mentioned by other mummies here, different kids progresses at different pace.
For me, my DD1 hardly crawled on all 4s but she can walk steadily by the time she turned 13 months.
Do see professional help early if you are too concerned or get your doubts cleared to get peace of mind.