How do you know when they're talking about a single child vs. multiple children here?
In frisian we have something which litterally translates to 'aunt-sayer' someone who says aunt to you, and 'uncle- sayer' someone who says uncle to you, to refer to the nieces and nephews .
I might be wrong, but Isn't nephew a gendered word, referring to a male? Whereas barn is ungendered.
Shouldnt it be 'mina brors' since we are talking abiut muktioke children and not just one child?
No. In this case, "min" refers not to the children but to the brother. If you had multiple brothers and all your nephews and nieces liked baking, you would use "mina" for the multiple brothers. But then you would wouldn't be using the singular possessive "brors".
Is „brorbarn“ („systerbarn“) a word, can I use it here instead of „brors barn“?
My brother's child is my nephew so you would not use three words in English when one will do!