1st? 2nd? Once Removed?
Think of cousin relationships in terms of sharing a common ancestor. All cousins share a common ancestor. What kind of cousin you are to someone depends on what common ancestor you have and how far back your pedigrees go before you get to that common ancestor. Forget about brothers and sisters and children of cousins for now -- it is easier to figure this out by finding the common ancestor. Here's how:
-o- When two people share the same parent, they are brothers and sisters.
-o- When two people share the same grandparent, but do not have the same parent (so we know that they are not brothers and sisters), then they are first cousins.
-o- When two people share the same great grandparent, (but do not have the same grandparent or parent), they are second cousins. In other words, if your closest common ancestor with someone else is your great grandparent and that ancestor is also their great grandparent, then you are second cousins. Third cousins would share the same great great grandparent. Fourth cousins have the same great great great grandparent and so on. To figure out what kind of cousin you are to someone, get out your pedigree chart and get out their pedigree chart and find your closest common ancestor on the charts and see what kind of grandparent that ancestor is to you and what kind of grandparent that ancestor is to the other person.
Here's how "removed" fits in --
-o- When two people have a common ancestor, they are cousins of some kind. If they do not descend evenly from their common ancestor, then they will have a "removed" relationship. Consider this example: looking at your pedigrees, let's say the closest common ancestor you share with another person is the great grandparent of one of you and that same ancestor is only the grandparent of the other one of you. This means that you both do not descend evenly from that person.
-o- To figure out the relationship, you take the closest one to the ancestor, the grandparent, and that makes it a first cousin relationship (remember people who share a grandparent are first cousins, even if that person is only the grandparent of one of you). Next, you subtract the difference in the generations: great grandparent minus grandparent makes it one generation difference. This means that you are once removed.
-o- Put both together and you have first cousins once removed (for someone whose grandparent is the great grandparent of the other person).
-o- Another example: if your common ancestor was the grandparent of one of you and the great great grandparent of the other one of you, you would still take the closest relationship, that of grandparent, making it a first cousin relationship and then subtract. Great great grandparent minus grandparent equals two generations. Thus, put it together and you have first cousins twice removed.