I read a book in the 80s about girl who is staying with relatives and there is a garden in the backyard. There is a crystal/glass ball. There might be a ghost. There are some illustrations (a little). In the end they are watching fire works I think. I remember her looking out upstairs window? Ball is glowing?? help!

This is likely Jane-Emily by Patricia Clapp:

Emily was a selfish, willful, hateful child who died before her thirteenth birthday. But that was a long time ago.

Jane is nine years old and an orphan when she and her young Aunt Louisa come to spend the summer at Jane's grandmother's house, a large, mysterious mansion in Massachusetts. Then one day.. . Jane stares into a reflecting ball in the garden—and the face that looks back at her is not her own.

Many years earlier, a child of rage and malevolence lived in this place. And she never left. Now Emily has dark plans for little Jane—a blood-chilling purpose that Louisa, just a girl herself, must battle with all her heart, soul, and spirit.. . Or she will lose her innocent, helpless niece forever.