A sister?

Lately we've been kidding Baz and Hadrien about having a sister, since the house we're planning will have three kids' rooms. Initially Baz was against having a sister, insisting that two of the rooms should be his. Lately, though, he's coming around, asking, "What will a sister do to me?" (Subtext: Will she be another kind of Hadrien?), or, "Will a sister obey me?" (Subtext: Hadrien doesn't, so will she?) He's also been asking, "Where will a sister come from?" (From God, we said, hence the prayer the other night for God to give him a sister.)

Tonight he asked me, given that there were six billion people, how many would it make if we had a sister. The only logical answer I could give, of course, was "Six billion and one."