Question

find the relative clause. if it is nonrestrictive, set it off with commas. if it is restrictive, submit the text without making any changes.
when we were kids, my eldest brother whose imagination knew no limits would entertain us for hours with his fanciful stories.

Explanation:

The relative clause "whose imagination knew no limits" provides extra, non-essential information about "my eldest brother" (we already identify him clearly as the eldest brother, so the clause is nonrestrictive). Nonrestrictive relative clauses need to be set off with commas.

Answer:

When we were kids, my eldest brother, whose imagination knew no limits, would entertain us for hours with his fanciful stories.