She is right to account for this variable. She is not accusing you. Women tend to multitask better than men, and they do it when in important matters, they look at the grand scheme of things and then take into account what unknown/thought variables could negatively "pop-up", and infiltrate her vision/plan as a mother withing your family unit dynamic. Sometimes men feel women are being negative when, after they have a basic vision in mind, they often only verbalize these possible negatives. Do not take it personally, it is DATA. And the only correct responce you can give her...is all reasons you can think of to prevent that variable happening. This will reassure her. It is a flat out FACT, that even kind, well meaning people who think they got it under control...interact with their own biological children "better" than step children. PERIOD! And dont fool yourself into thinking you are aware and above this. All you can do is just be humble and aware at all times. You'll see! Oh I also suggest studying gods instruction of husband/wife roles. Study Maslow's Heirarchy of Needs to make sure you cover all these (in concrete/abstract ways) in your children. And a "discipline" method by Dr Thomas W. Phelan titled 1-2-3 Magic:Encouraging good behavior/managing difficult behavior in children. Let me know if this helps.

I have a blended family and it was hard at first to let someone else into what I had held by myself for so long..It's hard to trust someone else will truely care and treat your child the same as their own.The dogs are just an analogy for the real question which is will you still love my child as much as "our" child.She also is tackling the question or thought of how to keep her son feeling like he's first.
Maybe you can do stuff together to help when the baby comes,say going to the store with Paul ,making dinner for the family.She will need lots of rest and helping with Paul will be a big help..
Good Luck and congrats;)