MGM isn't affiliated with Disney in any way, short of their naming partnership at the company's Florida theme park and that park's access to MGM's film library.
In fact, MGM has wanted to end that relationship for quite some time.
The Walt Disney Co., however, does anyone many other movie studios. Among these are Touchstone, Dimension, and Hollywood Pictures. Each of these has produced many movies that are PG-13 or above in their rating.
While PotC is their first PG-13 rated film released under Disney's own brand name, it's not the first non-G rated film. That honor, for anyone who's keeping track at home, goes to 1979's The Black Hole.
