Definately send hte invites to the step-sisters. If you are afraid if seems gift grabby just stick in a note "hey we know you can't make it, we just wanted to make sure you had the info in case plans change." so they realise why they are getting the invite.
As for the mom to be, if it's not a surprize, yes send her an invite, she'll probably want it for a photo album or scrap book, not to mention she's probably half out of her mind by now and having all teh details written in one handy place is always convienent. If it is a surprize party send her a fake invite or make fake but "nice" plans with her - nothing is worse then being the recipient of a surprize party only to show up underdressed with no make-up!