Is a blood related aunt considered a member of your immediate family?

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.