Sitting unsupported is a 7-9 month thing, as is crawling (7 to 10 months) so I wouldn't worry too much yet.
Developmentally they are 5 1/2 months, so they probs won’t sit for another month or so, developmentally my baby was 7 months when he started to sit but was actually 9 months old, I was told the baby was born early (before 36 weeks) they will be caught up by the time they turn 2
Try not to worry or pay too much attention to developmental milestones. Even with babies born full term there is such a range between when they achieve these things. Every baby is different and as mentioned above, it can take 2 years before preemies catch up. My son was born at 31+4 and didn’t start crawling until 10 months.
My baby is almost 10 months old was born at 31 weeks and only started sitting about a month ago, no sign of crawling yet but loves when we hold him in a standing position, don’t stress yourself out, they’ll learn it all eventually