。
She introduced me to her older brother.
Literal
She [topic-は] me [object-を] [respectful-お]older-brother [to-に] introduced [for my benefit-てくれた].
紹介する ('to introduce') uses を for the person being introduced and に for the person they're being introduced to. Note the honorific お~ on 兄さん — when referring to someone else's family member, Japanese uses respectful kinship terms (お兄さん 'older brother,' お母さん 'mother,' お父さん 'father') rather than the plainer in-group forms (兄, 母, 父). This grammatical politeness toward the listener's family is a subtle but consistent feature of Japanese kinship vocabulary. The benefactive ~てくれる adds the speaker's gratitude.