Explanation:

Correct option 3: was interested

The segment that contains the grammatical error is: "was interested"

Correct sentence - Neither I nor my sisters were interested in learning music.

When using "Neither...nor" with compound subjects, the verb agrees with the subject closest to it (this is known as the rule of proximity). In this case, "my sisters" is plural, so the verb should also be plural → "were", not "was".