No, you share no ancestor with your cousin's stepbrother so you are not related. You could call that person your cousin if you were close or if he had been in the step-relationship from a very young age and was not asserting his different parentage.
"Step uncle" is not a term used in English to describe a relationship. Presumably it could refer to the brother of a step-parent, or perhaps to the second husband of the sister of a parent, or the adopting parent of an orphaned first cousin.
There is no English word to describe the relation of you and your brother-in-law's cousin's step-son. Your brother-in-law's cousin is not related to you, so neither is his cousin, and certainly not a cousin's stepchild.
Your son and your niece are first cousins. Her daughter and your son are first cousins, once removed.
Your son and your nephew are first cousins to each other. Your son's daughter and your nephew are first cousins once removed.
Yes, of course your son and your great niece are related. They are first cousins, once removed, to each other. (Your great niece's parent, your niece or nephew, and your son are first cousins to each other.)
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.
You are each others cousins.
Your son and your third cousin are third cousins, once removed, to each other. Your son and your third cousin's son are fourth cousins to each other.
There is no such thing as a half second cousin. You are second cousins if you share one great grandparent. Being someone's cousin means you are related to them.There is a step-2nd cousin. A step-2nd cousin is the daughter/son of someone's step-mom's/step-dad's cousin. It could also be the step-daughter/step-son of someone's mom's/dad's cousin.Ex.:Rob has a step-mom named Mary. Mary's cousin is Johnathan. Johnathan's child is Maria. Rob & Maria are step-2nd cousins.Maria's dad is named Johnathan. Johnathan has a cousin named Mary. Mary has a step-son named Rob. Maria & Rob are step-2nd cousins.
They are second cousins
They are out of synch by a generation. They would be first cousins once removed.
Your sister's daughter and your son are first cousins to each other. Your sister's daughter and your son's daughter are first cousins, once removed to each other. Your son's daughter and your sister's granddaughter are second cousins to each other.