Your mother's cousin's son who was not adopted would be your second cousin (step-cousin.)
Our cousins are our mothers sisters son :)
Your nephew?
cousin
Nothing because it is your step father and his cousin If your stepfather were in fact your father, the son of his First Cousin would be your Second Cousin. If you are close to your stepfather, or were formally adopted by him, you might call that person your Step-Second Cousin. If you really like your stepfather, or his cousin's son, and want to assert a relationship, you could even call him your Second Cousin.
Step-brothers
First cousins, as you share a common grandparent.
Your cousin's son is your cousin, once removed, but it is probably best to call him or her by their first name.
cousin because your cousins son is your 2 cousin
step niece
Your aunt's son and your children are first cousins to each other.
Great great cousins.