I read a young adult book in high school that I'd like to find again.

The main male character has inherited the ability to turn invisible
through his uncle or his father.  Turning invisible is not a pleasant
experience.
I remember a rather risque detail about the book, probably why it
still sticks with me now.
Anyway, he has school chums who are twins and boy and girl.  The
friend remarks "it's just like looking into a mirror".  One of the
things that happens is that the boy who can be invisible is trapped in
bedroom under a bed when he thinks he hears that the school chums
(brother and sister) are going to have sex and do have sex.
(I swear this novel isn't dirty)
Anyway, near the end of the story we learn that it can never be proven
that this happened and that the girl much later died in a car or
boating accident.

Hello.

Your book is "Fade" (1988) by Paul Cormier.

See this description on Amazon.com:

"In Fade... 13-year-old Paul discovers an incredible secret gift: he
can become invisible. His long-lost uncle appears, to tell Paul that
each generation of the family has one fader, and to warn him of the
fade's dangers."
source: Amazon.com
http://www.amazon.com/exec/obidos/tg/detail/-/0440210917/


The disturbing detail that you remember is confirmed in this review:

" The first part is where Paul learns from his uncle Adelard the
nature of this gift/curse...
We are allowed to see some of the uglier things in life.  One is a
father's physical abuse of his son.  Another is the act of incest
between two new acquaintances, a brother and his sister."
source:
Book Review: Fade, hosted by westernwebsites.com
http://www.westernwebsites.com/fuhriman/e02-04.htm

search strategy:
"teen fantasy", invisible

I hope this helps.

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Clarification of Answer by juggler-ga on 22 Oct 2003 23:59 PDT

Oops. The author is Robert Cormier.  Paul was the name of the character.

Awesome!!!!  Thank You