Bilineal Cousin

A genetic cousin appeared in my sister's autosomal DNA matches at 23andMe.  The match illustrates how your DNA results can confuse your search, especially if you do not have other close family members tested.

The shared identical DNA between S. G. and my sister was two segments comprising 0.40%  This is worth exploring and could be as close as a third cousin.  Upon comparing other family members, we can see that the actual relationship will be more distant and from two separate ancestral lines.

23andMe:  DNA Relatives (viewed with Google Chrome Extension)
The M/P indicates that this cousin matches my sister through both parents.
(The M/P function only works if you have both parents tested at 23andMe and link them to the child.)

Cousin S. G. matches my sister through both our mother and father.  My sister inherited both segments from each parent, while I inherited none.  This is the randomness of DNA inheritance.

23andMe: Family Inheritance Advanced.
Cousin S. G. shares one segment with each parent.  My sister inherited both segments.

23andMe: DNA Relatives
Cousin S. G. matches my mother 0.25%

23andMe: DNA Relatives
Cousin S. G. matches my father 0.14%
The aggregate in my sister is thus 0.40%

So how is S. G. related to my parents?  I don't know.  Because close cousins have tested on both sides of my family, I can narrow down the possibilities.

23andMe: Family Inheritance Advanced
S. G. matches three members of my paternal family on the same segment.

S. G. matches my father and one of his siblings on the same segment.  More importantly, S. G. matches their mother's maternal cousin on the same segment.  This eliminates 75% of my father's family tree from holding the Most Recent Common Ancestor.  The match will be in my father's mother's mother's family tree.

I cannot narrow down the possibilities in my mother's tree, though.  S. G. does not match any of my mother's close maternal relatives.  This DOES NOT rule out a match on her maternal branch.  The common ancestral line could be anywhere in my mother's family tree.  This is much more difficult to work with.