You are first cousin, once removed to the children of your great uncle.
Your great uncle's children are your second cousins, because you share a great grandparent.
The children of your great uncle are first cousins to one of your parents, and first cousins, once removed, to you.
You are a great great Aunt (or Uncle) to the grandson of your Nephew.
Your relation to your mums uncle is GREAT uncle. You're welcome! 
Your niece's children are your great nieces and great nephews, and you are their great uncle. Your uncle could be the father of your niece, and thus the grandfather of your niece's children.
You don't, simple as that. You could say cousins if it's that important, or "my aunt's nieces and nephews on the other side," but there isn't an actual term for that except your "uncle's wife's siblings children."
The children are both your cousins[uncle's children]and your nephew and niece[sister's children].
Your neice's children are your grandneices and grandnephews. You are their great aunt or great uncle.
Great Aunt/Uncle
Your child's children are your grandchildren. Your great uncle's children are your parent's first cousins and your first cousins, once removed. Your grandchildren and your great uncle's children are first cousins, three times removed.
Your nephew's children are your great nieces (girls) and great nephews (boys). You are their great uncle or great aunt, depending on your sex.
It would be your second cousin because your mothers uncle would be your great uncle his brother would still be your great uncle because he is still your mum's brother so then his daughter would your mum's niece and your cousin.
'Great aunt' and 'Great uncle' are what your children properly call your own aunt and uncle. Your aunt and uncle are the brother and sister of your parent. Your parent is your children's grandparent.
A Great half uncle.