My sister laughed to her heart's content.

Literal

Younger-sister [topic-は] heart-go [until-まで] laughed.

心ゆくまで ('to one's heart's content') is a literary set phrase — 心 ('heart') + ゆく (classical form of 行く, 'to go') + まで ('until'). It means 'until the heart is satisfied.' More elevated than 思いっきり or 好きなだけ.