Cousins would share up to 50% of the DNA.
No. The daughter of your first cousin is your first cousin, once removed. The daughter of your second cousin is your second cousin, once removed. First cousins share grandparents. Second cousins share a great-grandparent; but not grandparents. Your niece is the daughter of your brother or sister. Brothers and sisters share parents.
You would share up to 12.5%. EDIT: NO, that would be a first cousin. A third cousin would share <1%.
Your first cousin.
Your first cousin's niece is the daughter of her brother or sister. Your first cousin's brothers and sisters are also your first cousins. So your first cousin's niece is the daughter of your first cousin and is therefore your first cousin, once removed.
One fourth
There is no relationship. You do not share a common ancestor. Your mother's cousin would be your first cousin once removed.
Your dad's cousin would be your cousin, once removed. The 'once removed' means that there is a generation difference between you and your dad's cousin. Your granddad's cousin would be your cousin, twice removed. The 'twice removed' means there are two generations difference between you and your granddad's cousin.
Any child of your first cousin, is your "first cousin once removed". Indicating they are 1 generation from being your first cousin. Your children & the children of your first cousin would be "second cousins".
Any child of your first cousin, is your "first cousin once removed". Indicating they are 1 generation from being your first cousin. Your children & the children of your first cousin would be "second cousins".
Your aunt's son is your first cousin. Their daughter would be a first cousin once removed.
Your father's aunt's first cousin could be your first cousin, twice removed. This would be the case if the first cousin is descended from your grandparent's grandparent. In addition to being your father's aunt's first cousin, this person would also be your grandparent's first cousin. Your father's aunt's first cousin could also be unrelated to you. This would be the case if your father's aunt is his aunt because she married the brother of one of his parents. Then her first cousin would not share any of your ancestors and would not be related to you.
Yes, for example if a first cousin of yours had a child it would be considered your second cousin.