。
She was angry at her younger brother.
Literal
She [topic-は] younger-brother [at-に] stomach [object-を] was-standing.
腹を立てる ('to get angry,' literally 'to stand one's belly up') is an idiom where 腹 (stomach/belly) represents emotions — a concept rooted in Japanese body-emotion associations where the belly is the seat of feelings. ~ていた marks the ongoing state of being angry at that time.