Things will work out. Patience and faith in your parents who have probably been there standing by you. This is a slightly bigger bomb for most people than annoucing gender preferences as being homosexual is much more "out" there than poly is.
They love you, they have seen you all together, they will remember how happy you all are as a family and then realize that if this is what it takes to make you happy, then they can accept it. Approval would be nice, but not necessary.
*hugs* for you all.
Life is about the journey and not the destination,
so what better way to know life
than to wander all the roads and paths set before you.