Second cousins once removed.
niece in-law
You are their great aunt/uncle.
3rd cousin
Nephews and nieces
2nd uncle I believe
A cousin, in all those difficult cases they are your cousins
niece/nephew
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.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
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