Nuns tend to get a bad rap in movies as strict, hand-slapping taskmasters who love to torment schoolkids.
Not our Reverend Mother.
We don't really get backstory on Maria's family, but she doesn't appear to have any around when we meet her. As far the movie is concerned, Reverend Mother takes on that maternal role for our heroine.
In many ways, she's the perfect parent. She doesn't want to force Maria to be someone she's not. When Maria doesn't seem to be cut out for the nun's life, Reverend Mother pushes her to try being a governess. She's not at all judgmental about Maria's behavior:
MARIA: Which brings me to another transgression, Reverend Mother. I was singing out there today.
REVEREND MOTHER: Only in the abbey do we have rules about postulants singing.
MARIA: I can't stop wherever I am. Worse, I can't seem to stop saying things. Everything I think and feel.
REVEREND MOTHER: Some call that "honesty."
When Maria tries to run away from her budding romance with Captain von Trapp, the Reverend Mother pushes her to face her fears/uncertainties and really figure out what she wants.
REVEREND MOTHER: Maria. The love of a man and a woman is holy. You have a great capacity to love. You must find out how God wants you to spend your love.
MARIA: But I pledged my life to God. I pledged my life to his service.
REVEREND MOTHER: My daughter, if you love this man, it doesn't mean you love God less.
REVEREND MOTHER: You must find out. You must go back.
MARIA: You can't ask me to do that. Please let me stay. I beg—
REVEREND MOTHER: Maria. These walls were not built to shut out problems.
Really, isn't that what good parents do—push their kids toward paths that suit them, and make sure they don't shy away from challenges that will make their lives better in the long run? We think so, and that's why Reverend Mother is definitely a mother figure to Maria, even beyond her role as "mother" to the abbey's nuns.