My sisters nephew would be considered what in relation to me?

I see that no one has answered this post.  I'm applying
what I know of family trees and inheritance to piece this together, but would welcome a
correction if there is an expert who reads this and finds my answer
incorrect.

To make this easier, let's give everyone a
name.

YOU = Tyson
YOUR SISTER =
Suzi
YOUR SISTER'S HUSBAND = John
NEPHEW =
Billy

If I'm not mistaken, Billy is absolutely nothing to
you... legally anyway.  Mary's connection to Billy is by marriage only therefore, yours
is marriage plus one more degree of separation (Mary).  Perhaps you could call yourself
Billy's Uncle In-Law, but even that, I don't believe, is technically correct.  Legally
speaking, if everyone in your family and Billy's family died except you - you would
probably not be able to claim Billy's inheiritance based on a family
connection.

Also, legally, you have no blood connection to
Billy.  If you had a daughter, she could marry Billy and it would not only be legal, but
genetically safe as well.

Now, if this is a brain-teaser,
Suzi and John's nephew, Billy, could very well be YOUR SON.  Or, he could simply be your
NEPHEW if he is the son of another sibling of your's and
Suzi's.

At any rate, the kid will probably never meet you
except at some random wedding or family reunion, in which case, he'll just call you
Uncle Tyson.  It has such a nice ring to it though, you might as well get ALL the kids
under the age of 10 at that wedding or reunion to call you Uncle Tyson - whether they
are relatives or not.