Had a long talk with DH about this and his main problem was with my sister, not my mum. He did say that it was a good sign on their part that they were making the approach rather than me. I also said he didn't have to stay if he didn't want to,which he did end up doing anyway.
So the visit itself, it was short but not short like last time. They stayed about two hours, we had afternoon tea and it went pretty well. I'm also going with not telling them more about the pregnancy than I'm prepared to share, particularly with my sister who is the doctor. DH did come in every now and again and sit with us, which was a nice surprise and he did help me tidy up a bit that morning as well.
We are okay having them over, not my stepfather, but as long as my mum doesn't like driving up she's beholden to him. But that's not something I can really do anything about as I no longer have any desire to have any relationship with him. She does want to come visit after baby has been born so if she still doesn't want to drive I can suggest train and bus to our place as it's not much trouble.
All in all, that went well.