You'll be great. It'll be a big transition for the MB too so I'm sure she'll appreciate feeling like you're both on the same side while this is happening.
It will change things certainly, but given your tenure with the family I'm sure it will be a wonderful experience. The bigger challenge might perhaps be in having MB home on maternity leave for a while - you'll probably look forward to it being just you with the kids, but in the meantime you and she can manage the transition together and figure out the new approach w/ multiple kids.
It'll be fun! The 20 mth old will suddenly seem so grown up (and will actually be able to be a little bit helpful) and the newborn will seem SOOOO easy and portable (and non-argumentative!!)
