"Should I Tell My Sister I Slept With Her Fiancee?" - Lady Seeks Help

During my wild days in the
university nine years ago I had
a brief fling with a guy I met
through a friend. We had séx a
few times and saw each other
for about a month before we
lost touch.
Fast forward eight years later; my
immediate younger sister
introduced this same guy to me
as her boyfriend. I was hoping
they would break up so he
wouldn't be an issue between
my sister and I but that didn't
happen.
A year later, they are getting
married. Their wedding is
actually this Saturday December
15th. There's serious friction
between the guy and I as we
can't even look at each other or
talk directly to each other.
I have been avoiding him since
he hooked up with my sister but
now that they are about to be
married, I'm stuck.
Should I tell my sister what
happened between us or leave
things the way they are? Does
she have a right to know?
I'm confused. Pls help!!!