Starting to understand about "baby sister" on the way?

I worry that Luke has no idea what is coming with this new baby on the way. He is pretty young, only 17 months, so it is hard to explain to him. Everyone keeps saying things to him like "you have a baby coming" or "are you excited for your little sister?"...and I can tell he has no idea what they are talking about. We try pointing to my stomach and telling him there is a baby inside but I think this confuses him more! But I think being at the cabin with our friends and their 10 month old baby might have helped. She is really not that much younger than Luke, but I think it was good for him to be around her because we really don't have any family or close friends with little babies. I kept saying..."do you want a little sister like Myah?" "Lets get a baby like Myah!" I think all my comments might have started sinking and maybe it clicked to him what a baby actually is and that we might be getting one because since we've been back he LOVES reading a little book I got him called "My New Baby." Maybe coincidence? Here are the two munchkins...