**Core Concept**
The question requires an understanding of diphtheria transmission and the role of booster vaccinations in preventing disease spread. Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae, which can be spread through respiratory droplets or close contact with an infected person.

**Why the Correct Answer is Right**
The correct answer is based on the fact that the sibling has received a booster vaccination 18 months ago, which provides immunity against diphtheria. However, immunity wanes over time, and the sibling may not be fully protected against the disease. Nevertheless, the sibling can still act as a carrier and potentially spread the disease to others. To prevent further transmission, the sibling should be given a booster dose of diphtheria vaccine, typically within 2-5 days of exposure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because administering a vaccine to the sibling is not the primary concern in this scenario. The focus should be on preventing further transmission of the disease, not just vaccinating the sibling.
**Option B:** This option is incorrect because there is no indication that the sibling is showing symptoms of diphtheria or is a carrier of the disease. Isolating the sibling would be an overreaction.
**Option C:** This option is incorrect because the sibling's vaccination status does not necessarily mean they are immune to diphtheria. The booster dose was given 18 months ago, and immunity may have waned since then.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that booster vaccinations can wane over time, and individuals may not be fully protected against diseases even if they have received the recommended vaccinations. In situations where there is a high risk of disease transmission, booster doses may be necessary to prevent outbreaks.

**Correct Answer:** C.