That won't work. The earplugs will mask the rumble of the car and force her to hear the music better than ever. Not to be alarmist, but justifiable homicide, with Squeak as the victim, could be the result. This depends, of course, on how the sister reacts to Liz's kind of music.
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Once my husband and I were riding in his sister's car. She said, "Have you ever listened to Bruce Springsteen?"

"Not really," we said, meaning, hell no.

She then turned on the most godawful, rackety irritating stuff, but we didn't say a word, listening and trying to give the guy a chance. Pretty soon, however, she saw the white knuckles and clenched jaws, and she turned it off. If she hadn't, who knows what would have happened.