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Alas, I've never quite gotten the hang of grouping, so I don't do this in the elegant way suggested by my colleagues.
Bring up the one you want to be the surviving individual in the individual record screen. Put the cursor in the name field. Hit the "Find" key. It won't find her, of course. Enter her name as Mary /Smith/ in the "Find Again" field - do include the slashes. When it finds her hit "Identify"
Then scroll down the list of individuals until you come to the one to be merged, e.g., Sister /Smith/. If there's more than one, you'll need to know the number. When she's selected, hit the "Esc" key.
The dialogue will ask if you really want to do this - hit "yes" and it's done. Ordinarily you can choose to delete the merged individual. Then delete the duplicate birth, death, marriage, etc. events for the surviving individual.