In this sentence about singing siblings, choose the pronoun that best completes the blank: “It so happens that when siblings sing, one of them invariably has a less potent vocal prowess even though ______ have a similarly commendable music base.”

Introduction / Context:
This English grammar question focuses on the correct use of pronouns in a comparative sentence. The sentence talks about two siblings who sing, and contrasts the weaker vocal prowess of one with the strong musical foundation shared by both. Your task is to select the pronoun that fits grammatically and logically in the blank.

Given Data / Assumptions:

1) The sentence begins with “when siblings sing, one of them invariably has a less potent vocal prowess”.
2) The next clause reads “even though ______ have a similarly commendable music base”.
3) The options are: both, their, you, we.
4) The subject of the sentence is “siblings” (two people), and “one of them” refers back to that pair.
5) The blank must refer to the same siblings and match the verb “have”.

Concept / Approach:
Pronoun choice depends on agreement with the noun it replaces and on sentence structure. Here, “siblings” is plural and refers to two specific people. The clause “even though ______ have a similarly commendable music base” needs a plural subject that clearly refers back to the siblings as a pair. We must also check which option can naturally follow “even though” and precede “have”.

Step-by-Step Solution:
Step 1: Identify the noun being referred to: “siblings”, which is plural and specific. Step 2: Note that the verb after the blank is “have”, which requires a plural subject (“they”, “both”, etc.). Step 3: Test “both”: “even though both have a similarly commendable music base” is grammatically correct and clearly refers to both siblings. Step 4: Test “their”: “even though their have a similarly commendable music base” is ungrammatical because “their” is a possessive determiner, not a subject pronoun. Step 5: Test “you”: “even though you have a similarly commendable music base” suddenly addresses the reader or listener, which breaks the focus on the siblings. Step 6: Test “we”: “even though we have a similarly commendable music base” wrongly includes the speaker and changes the meaning. Step 7: Therefore “both” is the only option that agrees in number, keeps the reference clear and preserves the original meaning.

Verification / Alternative check:
Read the full sentence with “both”: “It so happens that when siblings sing, one of them invariably has a less potent vocal prowess even though both have a similarly commendable music base.” This shows a clear contrast: one sibling sings less powerfully, but both share the same strong foundation. The sentence is smooth and logically consistent, confirming that “both” is correct.

Why Other Options Are Wrong:
“their” is a possessive determiners and cannot serve as the subject of the verb “have” here. “you” would shift the focus to the reader, which is unrelated to the earlier “siblings”. “we” would include the speaker and again disrupt the sentence logic.

Common Pitfalls:
Students often choose “their” because it seems to refer to the siblings, but they forget that subjects need subject pronouns or noun phrases, not possessive determiners. Another pitfall is ignoring viewpoint; pronouns like “you” and “we” signal the speaker or listener, and should not be used when the sentence is clearly about third persons “siblings”. Always check that the pronoun matches both number and perspective.

Final Answer:
The pronoun that best completes the sentence is both.