No, that would be "Ich sehe eine Nonne." As "die/eine Nonne" is "the/a nun" in German. :) "Nonnen" are "nuns", btw.
"I see a sister of the bride"
"Can you see any of the brothers?" "...I see a sister"
Which is more the most common translation - a nurse, nun, or sister? From this context - it's really hard to tell for me.
It depends on whether you're at a birthday party, in a hospital or in front of a nunnery. If you'd do some frequency statistics on the whole German language, the order (from highest to lowest) would probably be: sister, nurse, nun.
It's correct but without context I wouldn't consider it a very natural translation