A relationship with a parenting is when you get into a relationship with someone who has a child
no
No, there must be a blood relationship.
Yes
your grandchild
yes
first cousin twice removed
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Your son's child and your nephew are second cousins.
Your cousins child is your first cousin once removed.
The relationship between your sister's (or brother's) child and your cousin is the same as the relationship between your child and your cousin. Your child and your first cousin are first cousins, once removed. Your child and your second cousin are second cousins, once removed, and so forth.
He is your nephew and you are his uncle.