Your cousin's sister is also your cousin.
Your first cousin's children and you children are second cousins.
Your second cousin's children and your children are third cousins.
And so forth.
He is your second cousin once removed. His children are your third cousins.
Second cousins.
The children of your mother's first cousins are your second cousins. The children of your mother's second cousins are your third cousins. And so forth.
Children of sisters are first cousins. Grandchildren of sisters are second cousins. Great grandshildren of sisters are third cousins.
your children are allowed to Mary your second cousins.your children are allowed to Mary your second cousins.
Your second cousins.
Your mother's sister's children are your 1st cousins.
Your children and your first cousin's children are second cousins. Your children and your second cousin's children are third cousins. And so forth.
Your son and your nephew are First Cousins. The children of your nephew and the children of your son are Second Cousins to each other.
Cousin once removed.
Your uncle's grandchildren are your first cousins, once removed. They and your grand children are your second cousins, once removed
Let's assume you are talking about first cousins.If they are children of the same cousin they are siblings (brothers and sisters).If they are children of different cousins and the cousins are both children of the siblings of one of your parents, then the cousin's children are second cousins to each other.If one of the cousins is a child of a sibling of one of your parents and the other cousins is a child of a sibling of your other parent, then the cousin's children are not related to each other.If you are talking and the children of second cousins, they children are siblings, third cousins or not related.If you are talking about cousins who are once, twice or more removed from you, the possible relationships become too complex to discuss here.