Hey man, there is no "official" protocol on any dating and can't be!
But in your case, it's just obvious that you wouldn't mind to fuck her and give her a great chance to have another little baby from an unknown father!

Obviously you don't have an idea what a little baby is about, she needs to give baby her tits to make him happy on food, if you suck such tits, you will be surprised as they could be full of quite nasty milk!
I think you don't have to ask for the permission to anyone, both the mother and brother may appear stupid as this girl, so take your chance and make her happy as deep as you can!