Congrats. I have a 5yo step daughter and my DF & I are now expecting our first together. We told her quite early on - I think at about 12 weeks. She has been very excited about it all. We have tried to include her as much as we can. She went on the hospital tour with us, went to the first midwife's appointment, comes along on baby shopping expeditions and gives her input.

In general she's very happy about it all and talks to her little brother inside my bump all the time and "introduces" him to people all the time. She has recently started to get a little more babyish in her eating so we are having to put some extra effort in there. I've read that this is perfectly normal, so depending on what age your step kids are you might experience this. We also regularly get "you love the baby more than you love me" and we have to reassure her that this is not the case. The only other thing I can think of is that when we are in toy departments she'll say stuff like "the baby is getting all these new things and I get nothing" (which isn't true, she's quite spoilt actually) so we just explain that he's going to be new to the world and doesn't have anything yet so we have to buy him what he needs, whereas she already has these things.

It's going pretty well so far. I'll let you know how it goes after the birth, could be a different story altogether.