If the great aunt of person A and the grandmother of person B are sisters, then the A and B are either siblings themselves, or they are first cousins or second cousins to each other.
First cousins are relatives who have the same grandparent. Second cousins are relatives who have the same great grandparent Third cousins are relatives who share the same great great grandparent. If you and a relative have great grandparents who were brother and sister, then you share a great great grandparent, and are third cousins.
If someone is your fourth cousin, their great great grandparent was a brother or sister of your great great grandparent.
Your cousin's cousin may be you, your siblings, your cousin, or may not be related to you at all. Your cousin is related to you by being descended from the brother or sister of your parent, grandparent, great grandparent, etc. Your cousin will have cousins who are also descended from those people, but will also have cousins on the other side of her family who have no relationship with you.
If Joe's grandfather and Jane's great aunt were first cousins, and Jane's great aunt was the sister of her grandparent, then Joe and Jane are related as third cousins. But if Jane's great aunt is married to the brother of her grandparent, Joe and Jane are not related.
A great great aunt is the sister of your great grandmother. (A great aunt would be the sister of your grandmother.)
Two people with the same great uncle may be first cousins or second cousins to each other, or may not be related to each other at all. It depends on whether the great uncle is the brother of a grandparent, or the husband of a sister of a grandparent.
Your brother's wife is your sister-in-law, but her sister is not related to you.
Your sister-in-law is either the wife of your brother or the sister of your spouse. Your brother-in-law is either the husband of your sister or the brother of your spouse. So your brother-in-law's wife is either your sister or is not related to you. Some people incorrectly refer to a brother-in-law's wife as a sister-in-law.
The sister of your grandmother is your great aunt The daughter of your great aunt is your parent's first cousin and your first cousin, once removed. Her husband is not related to you. A few people might try to call him your first cousin, once removed-in-law but most would find that uselessly complex and would call him, "the husband of my first cousin once removed," or "my cousin's husband."
Not everyone in the world is related or brother and sister. Some people may feel that they are all related i the religious or biblical sense but not legally.
If your brother-in-law is the brother of your spouse, hisbrother is also your brother-in-law. The daughter is your spouse's niece and therefore your niece as well.If your brother-in-law is the husband of your sister, his brother is not related to you, as the cultures based on the English language define family connections, but is your sister's brother-in-law. His daughter is also not related to you but is your sister's niece by virtue of her marriage to your brother-in-law, who is the daughter's uncle.
Since your great grandfather was brother to the other person's great grandfather, the two great grandfathers had at least one parent in common. That parent is your common ancestor, the great great grandparent of both of you. People who share a grandparent are first cousins. People who share a great grandparent are second cousins. People who share a great great grandparent are third cousins. You are third cousins.