Don't sweat it. You get what you get. If you get too much, pass it on along to someone else who needs it. There will be another mom in your church that will have a baby and you'll be in the position to bless her.
Plus, the 0-3 month clothing you have might not be the right season. You just never know.
Plus: Will your brother just continue to hand-me-down clothes? So won't you be in this same position with the 6+ month clothing?
Just smile, say thank you, return what you can that you really don't need and bless others with your extras.