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I can't speak to the step parent issues, but it sounds like JamStuck has a good read on it. But as an observer (I've coached hundreds of kids in baseball and soccer) it is my opinion that the step parents who make out the best are the ones who accept their step kids for who they are rather than who their other parent is.
If you are concerned about your health and the health of your baby, the best person to talk to is your OBGYN. They have a LOT of experience with all things pregnancy!
Best wishes and gentle hugs
Your little girls might be too young for that but you and your fianc and maybe even his ex going would help. Or at the least to sit down and establish some ground rules. It's only going to get more complex when the baby comes.
Yes, little kids do get sick often. Guess they are just preparing you for when you have your baby.
Like Iamstuck said, take it easy and try to get some rest.
Sending you support and hugs.