"First cousin, once removed" describes an asymmetrical relationship, so it's not possible to answer definitively. The person in question could be either your second cousin or your first cousin twice removed.
Your mother's first cousin's child is her first cousin once removed and your second cousin.
Your mother's great-uncle's (or great-aunt's) child is her first cousin once removed and your first cousin twice removed.
I think they are you cousins once removed
First cousin, once removed.
Your mother's first cousin is YOUR first cousin, once removed. The child of your mother's first cousin is YOUR second cousin.
Your mother's aunt's son is her first cousin and your first cousin, once removed.
The child of your first cousin, once removed, is your first cousin, twice removed, if your first cousin once removed is the child of your first cousin. If your first cousin once removed in the first cousin of one of your parents, the child is your second cousin.
If your mother's cousin is her first cousin (if one of each of their parents were siblings) then that cousin is your 'first cousin once removed'. If that cousin has any children, they are your second cousins.
If your first cousin once removed is your parent's first cousin, then your first cousin once removed is your child's first cousin twice removed.If your first cousin once removed is the child of your first cousin, then your first cousin once removed is your child's second cousin.
Your parent's first cousin is your first cousin, once removed. Your parent's first cousin, once removed is your first cousin, twice removed. Your parent's second cousin is your second cousin, once removed. Your parent's second cousin, once removed is your second cousin, twice removed.
Your 1st cousin's child is your 1st cousin once removed. Your 2nd cousin's child is your 2nd cousin once removed. And so on...
Your mother's first cousin's child is your second cousin. To your children this person is a second cousin once removed.
Your father's first cousin, once removed, is your 2nd cousin. Your father's second cousin, once removed, is your third cousin.
The son of your second cousin is your second cousin, once removed. You are also his second cousin, once removed.