She gently held her younger brother.

Literal

She [topic-は] gently younger-brother [object-を] embraced.

優しく弟を抱きしめた — 優しく (やさしく, 'gently, kindly') is the adverbial form of 優しい. 抱きしめる (だきしめる, 'to hug tightly, embrace') is a compound verb from 抱く + 締める.