My understanding is that they USUALLY sit before crawling and so sitting is seen as a developmental step. But crawling is not seen as a milestone because some babies never learn to crawl.
Although most sit before crawling, the ones that don't will sit once they are crawling if that makes sense - Im pretty sure crawling is a milestone they watch for because of the cross over of hands once they start is important for brain development. If babies don't crawl, then once they are walking it is important to do activities where they cross over their hands and play crawling games so their brain can make these connections. I'm not sure I've explained it right, but Kimnastics posted in one of my threads once when I was worried, will see if I can find it!

I agree with Ren too - babies seem to do what they want to do, in their own time, rather than the time we want them to,so go with the flow, don't rush it and you will be fine!!