My DS was the same, he was weightbearing and trying to walk with us holding his hands at only 4 months. Commando crawling at 6 months and pulling himself up and cruising around the furniture by 8 months. Everyone thought he would be an early walker and skip crawling but he asn't. He eventually started proper crawling at 11 months and walked at 14 months.

Don't worry about it (lol says the mum who did worry!). She will do everything in her own time. And bad news for your MIL, you can't stop babies from walking anymore than you can MAKE them walk. If they are ready, they're ready and if they're not, they won't.

And my DH was a bottom shuffler too, never crawled and he is far from slow, believe me.