"Yes, my love, one day when you're older, you will have boobies. But by then Baby Brother won't need boobie milk any more so you won't be able to feed him".

Oh wow this conversation was cute. She so dearly loves her brother and loves the idea of being able to breastfeed him. One of those things that's so adorable, I would usually write it down to share with them later... But I don't think the idea of one sibling breastfeeding the other is ever going to elicit an "awwww" from either of them. Just an "ewwwwwwww!!!".