I've figured it out by consulting multiple calendars and gender charts. I'd like to conceive Declan and B.B.'s little sister in mid-late May 2015, so that she's due in early February 2016. That way Baby Bro and Declan can both be in school (well, preschool at least) and I can have some time to myself.
I know this is silly, but like I just did a bunch of work to figure that out (just for curiosity's sake) and I wanted to write it down.