This is for everyone who gets as confused as I do, when a group of relatives tries to explain this!
Your first cousin is the child of your parent’s sibling. Your second cousin is the grandchild of your granparent’s sibling. And so on.
The way to remember it is how many generations away the bridge is. For instance, with my first cousin Kevin, the bridge is that his mother is my mother’s sister – since my mother is one generation away from me, he is my first cousin. And with my second cousin Melanie, her grandmother is my grandfather’s sister. Since my grandfather is two generations away from me, she is my second cousin.
Easier way to explain:
Each case refers to natural uncles and aunts.
First cousins- your uncle’s children and your aunt’s children
Second cousins- the children of your uncle’s children and aunt’s children
Third cousins- the offspring of the children of your uncle’s children and aunt’s children
Now for the tricky part:
Removed: When the word "removed" is used to describe a relationship, it indicates that the two people are from different generations. "Once removed" indicates a difference of one generation, "twice removed" indicates a difference of two generations, and so forth.
For example, the child of your first cousin is your first cousin, once removed. That is, your cousin’s child would be "almost" your first cousin, except that he or she is one generation removed from that relationship. Likewise, the grandchild of your first cousin is your first cousin, twice removed (two generations removed from being a first cousin).
Many people confuse the term "first cousin, once removed" with "second cousin." The two are not the same.
I just think of the "once removed" or "twice removed" meaning how many leaps up the generational ladder you have to get to the level of cousinship. My first cousin twice removed would be my grandmother’s first cousin (two generations back from me).
Your first cousins once removed are the children of your first cousins. If you have children, then they will be the second cousin of your children.