What does Bruno realize about Maria? How does Maria's revelation about her past change Bruno's opinion of his father?

I would say in a
nutshell, Bruno understands that Maria is a human being. She is not an extension of his needs,
but a fully formed person with her own past, needs, wants and desires. 

You
could say in some regards, this is also what Maria's insight into her past helps Bruno see about
his own father. This is a common part of development in adolescents, as they learn to accept
their parents and other family members as individuals and not just as part of their
lives. 

There is some more info on the Bruno/Maria relationship in the source
link provided below.