Is the warranty in effect from the builder? Here is how it should be fixed:
FAQ About Old Plaster
Is it wire lath under or what? I'm thinking when installed, it wasn't re-enforced at the corner for just this.
Is there a beam centered on the opening below, with the right side dropping?
Put a 6' level (or straight-edge and 4' level) on the floor, across the arched opening. Is the floor level? Be safe, G