Collateral ancestor is a legal term referring to a person not
in the direct line of ascent, but is of an ancestral family. This
is generally taken to mean a brother or sister of an ancestor (hence a
"collateral ancestor" is never an ancestor of the subject).
Collateral descendant is used to refer to a descendant of the brother or sister of an ancestor.
The general way of referring to a blood relative who is not an ancestor or descendant is as a collateral relative.
Cf. Allied Family