If you are a female you could be talking about yourself. You could also refer to a sister this way. This could also refer to a cousin of yours, since your aunt's husband may have nieces through more than one sibling (either his siblings or your aunt's siblings). It is not clear whether you are related to your aunt or to your uncle by blood, but it is very probably one of them and not both.
If your aunt is the sister of one of your parents, then her husband (your uncle) may have nieces who are not related to you at all. These would be the daughters of his siblings.
You would be a first cousin by marriage, in other words, there is no direct relationship between you.
Second Cousin
Great-niece.
your 2nd niece...!
Nephews and nieces
They are your nieces and nephews.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
He is their Uncle and they are nieces and nephews.
They would be great nieces or great nephews.
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
They are your great nephews and/or great nieces. Alternately, some people say grand-nephews, grand-nieces.
It says what relation are your brothers nieces to you.... It could be your child or your niece also causes you can have two brothers and it may be the other ones kids. Or of course it could be your brother's new wife kid by another man making her no relation.