Your dad's nephew is your cousin. Therefore, your dad's nephew's child is your cousin once removed. This means that they are one generation below you in your family tree.
Second cousins once removed.
niece in-law
Nephews and nieces
3rd cousin
2nd uncle I believe
You are their great aunt/uncle.
A cousin, in all those difficult cases they are your cousins
niece/nephew
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
Your uncle-by-marriage's nephews might be you and your siblings. You are your uncle's nephew or niece.They might be the children of your aunt's siblings (your aunt being his wife), in which case they are your cousins.If they are your uncle's nephews by virtue of being children of his siblings, they are no relation to you at all.
you would be the child's great aunty or uncle Therefore, they would be your great niece or nephew.
He would be his great great great uncle