you are still are considered an aunt or uncle. But, if you are to introduce them to someone outside the family then you are to be considered their great aunt / uncle
Great nephew
your nephews
Nephews are the sons of your brother or sister. Therefore nephews who are not brothers to each other are first cousins to each other.
3rd cousin
He is their Uncle and they are nieces and nephews.
Second cousins.
Most families would use the first name. There is no real relationship with them as there is no common ancestor. But some families might refer to them as cousins or even nephews, though they aren't.
Your brother's children would be your nieces and nephews.
Your brother-in-law's children are your nephews and nieces.
You would be his Great Uncle and he would be your Great Nephew.
Either a step-nephew or a son. "Step-nephew" is not a widely accepted or used relationship term but implies that someone's sibling has married a person who has a son from a previous marriage. That son is the step-son of the uncle's sibling, if the sibling is the person who is involved in the marriage. Otherwise the uncle has more than one sibling and you have another step-nephew relationship.
Second cousins once removed.