。
She played a prank on my younger sister.
Literal
She [topic-は] my younger-sister [to-に] prank [object-を] did.
いたずらをする ('to play a prank, pull a trick') is the standard collocation for mischief, with the prank-recipient marked by に (the target/recipient). いたずら itself is usually written in hiragana, despite having the kanji 悪戯 ('bad play') — a common pattern for words whose kanji are technically rare or unfriendly. Note that いたずら can also be a euphemism for sexual assault in heavier contexts, so register matters; in normal childhood-prank scenarios it's harmless mischief.