Yes, your niece is the daughter of your sibling (sister or brother), so there is continuity of "blood" (i.e., genetic descent) from your common parent(s), assuming your sister/brother and her/his daughter are not adopted.
If the nephew is a child of one of the aunt's siblings, then the nephew is blood related to his aunt. However, a nephew may also be the child of the sibling of the aunt's husband and is then not a "blood relative" of the aunt.
To the extend that the obsolete and rather meaningless term "blood relative" applies an aunt is a blood relative because an aunt is the sister of your mother or your father.
On the othe rhand, we also call the wife of an uncle (the brother of your mother or father) an aunt, and that person is not a "blood relative" because she shares no ancestors with you.
Aunts generally have visitation rights only when specified in a court decree.
Your brother's daughter is your niece, her daughter is your great niece and her granddaughter is your great great niece. It works the same for nephew, aunt, and uncle.
The opposite relationship of uncle would be Niece or Nephew.(The opposite gender is aunt.)
Your aunt is great great aunt to your son's daughter (your granddaughter). Your granddaughter is your aunt's great great niece.
Your uncle's wife is your aunt.
Aunts generally have visitation rights only when specified in a court decree.
A niece is the daughter of your brother or sister and therefore has at least one grandparent who is also your ancestor. Sharing an ancestor, you are blood relatives - to the extent that term has any real meaning. (Remember, we share DNA, not blood, with our relatives.) However, if the niece, or her parent, was adopted, she is still a niece but shares no genetics with you and so would not be considered a "blood relative" by those who use that term.
agnate, brother-in-law, aunt, cousin, blood
Your niece will call you her aunt or uncle.
aunt
Yes. You are the aunt of your sister's children and anyone you marry is their uncle.
Your brother in law's daughter is not a blood relative; however, by marraige she is your niece. She is the neice of your wife. Any relative of your wife becomes a relative of yours when you become "one" in marriage.
That's easy cousins.
Once the baby is born, you are considered to be its grand aunt or grand uncle.
If you marry your girlfriend, you niece (daughter of your brother or sister) will continue to be your niece and will also be considered by most to be your new wife's niece. Your new wife will be her aunt, as you are already her uncle.
Your Aunt's niece is you (if you are female), your sister, or your First Cousin.
You are her great-aunt/uncle. She is your great-niece.