I was told this as well with DD, she did a roll up to sitting then roll thing until she was 12 months old. She then started to crawl at 12 mths and was walking at 14 mths. As for her speech she has never had any issues and at the ripe old age of 2yrs 3mths she can hold a conversation with you, will repeat word for word scenes from Dorothy the Dinosaur and the Wiggles, knows her colours, body parts etc. So I don't think the delayed crawling affected her in any way, so long as they learn it at some stage for a little bit.

As for the twins, Liam started crawling just before they were 7 mths old and Rhys has started the past couple of weeks (8 1/2 mths) and both of them have been very 'talkative' babies from early on. Only time will tell with them I suppose.