Original: Paterson had asked his mother, Katherine Paterson, if he could write a screenplay of the novel, and she agreed "not only because he's (her) son, but also because he's a very good playwright".
Simplified: Paterson had asked his mother, Katherine Paterson, if he could write a screenplay of the novel. She agreed "not only because he's (her) son, but also because he's a very good playwright".