The Glass Between Us

Casey Evans was named after her great grandfather- word for word. The doctors said he committed suicide- but the family seems to have a secret theory, which they refuse to tell Casey.
Casey's father died when she was young, and her mother worked most of the week as a nurse. Her older sister, Jackie, was about to go off to college.
When Jackie commits suicide, it seems that the entire family shows up to town- all of them whispering nervously.
Casey is devastated, and now thinks she is going insane from loosing her sister.
Then, suddenly, a boy is in the mirror- where her reflection should be.
What is going on with her? Is she actually going insane? Did her sisters death have something to do with the huge secret in the family?
And why did she see that boy kill himself?