How do they solve a problem like Maria? According to which of them you ask, she's a "flibbertigibbet, a will-o' the wisp, a clown." Basically, they don't know what to do with her, but they love her anyway. When the children try to visit Maria at the abbey, the nuns know something's wrong and run to get the Reverend Mother. They're thrilled when she finds her purpose in life and marries the captain.
Plus, they're lifesavers: When Maria and the rest of the von Trapps come to the abbey to hide after fleeing the festival, the nuns hide them and even pull parts out of the Nazis' cars to prevent the soldiers from pursuing the family. You can see how deep their faith is; they stand up for their principles even though it's dangerous.