My DS could sit, if I put him in a sitting position for about 10 secs at 6 months, but he had no interest in it. He wanted to crawl, and preferred to be on his knees rocking and trying to move...
He was confidently crawling around 6.5-7 months, and within a week of him crawling, he would sit alone, unassisted. It was kind of a side sit for a bit, but it became a good sit quite quickly.
I was quite worried too, but turns out my worry was for nothing...