Yes, you and your great uncle are descended from the same people, two of your great great grandparents, so you are biologically related, but for most purposes you are not "close relatives."
Your mother's brother is your uncle.
If she is your biological granddaughter (not step or adopted), yes.
If by "stepson" you mean someone who was legally adopted into the family - yes, they are a legal relative. However, they are NOT considered to be a "blood" relative.
The only way you are a BLOOD relative is if you have the same blood line. You can't change a persons biological parents. If the step father adopts the child, then the child is by law their child and a family member, but not a literal blood relative.
No. Descendants are your biological children, grandchildren, great grandchildren, and so on. Some people call these blood relatives, but not all your blood relatives are your descendants. For example, your first cousin is a "blood relative" but not your descendant. On the other hand, everyone is descended from someone, so everyone is a descendant - just not yours.
Yes, especially if someone in the family (particularly the mother, or other biological relative) carries that blood type or a similar blood type.
While a son-in-law is a relative, he is not a blood relative. Neither would be a daughter-in-law, step-father, step-mother, step-brother, step-sister, or the spouse of a 2nd marriage to one of your relatives. Blood relative means another person shares DNA within a group of related persons. So typically, your direct genetic and genealogical (family tree) line would go from grandparent to parent to you. All of your grandparents' children (your parents and their siblings), your siblings, and your own children (when you have children) are blood relatives. An adopted sibling would not be a blood relative to other biological siblings.
Yes, a biological family member is one that you are related to by blood.
A blood relative is a person who is related to another through a common ancestor, not by marriage or adoption.
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.
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What?
Assuming the question relates to a child born out of wedlock, blood relations of the biological mother do not have more 'rights' to custody of a minor child than the biological father. In such cases, the court decides who shall retain permanent custody or joint custody of a minor. Generally the court will grant temporary custodial rights to the biological grandparents or the closest living relative of the biological mother assuming such person(s) qualifies for guardianship.