Every musical based on a movie that your relative have already seen can be a good choice (Lion King, Aladdin, School of Rock).I was myself a non-English speaker when I fall in love with musical theater.
I was 13 and I listened Les Misérables (TAC). I didn't understand most of the lyrics, but the music was so powerful that I got most of it (and a basic knowledge of the story help a little). The next year, I was listening The Phantom of the Opera and Miss Saigon.
If you're not afraid to spoiled some element of the story, you can also prepare your relative to the some elements of the plot (characters, setting, storyline). Or, pre-translate to your relative some of the catch phrases of the show. For example, If you see Wicked, you can tell her what "Dancing through life", "Defying Gravity" mean (but before the show, because during it's really annoying). Then, during the intermission, you make sure she understood the important point of Act 1. I used to do these things when I bring a non-English speaker to see a musical with me. It's work well with show like Lion King, Shrek, Billy Elliot, Phantom, Wicked,
Maybe I do not speak English as well as you, but I can perfectly pronounce the names of all characters of Les Misérables.