Question

What do nuns symbolise in a catcher in the rye

Answer 1

The nuns are a foil for Sunny--that whole mother contrast. Holden is looking for a connection with anyone, yet he doesn't find it with either side of that equation.  

Holden has regard for others? Yeah. So what? That doesn't contribute much to Salinger's purpose here. Holden seeks connection. He also seeks to find (and preserve) something pure in a corrupt world. That should be the focal point of your analysis of his interaction with these types of characters in the novel.