What is the correct superlative form to use when comparing multiple siblings?

The superlative form is used to compare three or more items or people, indicating the highest degree of a certain quality. In this case, when comparing siblings regarding the quality of being fun, the correct term is "the funniest." This superlative form is derived from the base adjective "funny" by following the standard rule of adding "-est" to short adjectives that are one syllable or using "most" for longer adjectives. However, "funny" as a base adjective is an exception where "funniest" is the accepted superlative form.

Using "the funniest" clearly conveys that one sibling possesses more humor than all the others in the group, making it an effective choice for the comparison. The other options either use incorrect forms or do not apply the superlative rule appropriately, which is why they are not suitable in this context.