YA Fantasy/ Coming of age/ Magic. I read this in high school and can't remember the name. Read in early 1980s. Spoiler.

[Darleen]

I read a book back in the early 80s about a boy and his friend that go to live with an eccentric and sometimes cruel family member (uncle or grandfather) for a summer. The house is very mysterious and one of the boys falls in love with a mysterious girl down by the lake which makes the friend jealous of the time he spends with her and not him. In the end the boy is turned into a glass bird by the family member and the friend takes him to the water to be with the mysterious girl. Please help if you know what book it is.