Your uncle's sister could be your mother. Then her niece is your first cousin.
Your uncles' sister could be a sister of either your mother or your father, and thus your aunt. Then her niece could be you (if you are a girl), your sister, or one of your first cousins.
If your uncle is your uncle because he is married to the sister of you father or your mother, then his sister is not related to you, and neither is her niece, unless the niece is the daughter of that uncle with the sister of one of your parents. Then the niece wold be your first cousin.
Your brother's nephew is always your nephew, unless your brother is not really your brother.
Your children and your nephew are first cousins to each other.
Your nephew's first cousin may or may not be related to you, depending on how the cousin is related to your nephew. Your nephew's cousin may be your child, or the child of one of your brothers or sisters. Your nephew is related to you by being the son of your sibling. That sibling has a spouse who may also have borthers and sisters. The children of any of those brothers and sisters are also your nephew's first cousins, but are not related to you.
It depends on which "half" of your half-brother the nephew is related to. Your half-brother shares your father or mother, but not both. If your half-brother's nephew is related to the same parent you are related to, your half-brother's nephew would also be your nephew. If related to the other parent, he would not be related to you.
Your nephew's nephew is the son of one of your nephew's brothers or sisters. He is your great nephew.
Your uncle's nephew could be:you, if you are a boyone of your brothers, if you have anyyour first cousin, if your parent and your uncle have a third sibling who has a child who is a boy
Nephew...
Nephew
Nephew.
Your Nephew :)
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nephew
Your brother's (or your sister's) son is your nephew and you are his aunt if you are a female or his uncle if you are a male.
Does the stepchild and your child share a parent? If so, they are half-brothers, which would make the grandchild your son's nephew.