。
Before her marriage, she was a member of the Brown family.
Literal
She [topic-は] before-marriage [topic-は] Brown family [possessive-の] person was.
The double は (彼女は…結婚前は) is doing two different jobs: the first is the main topic marker, the second is contrastive — 'as for before her marriage, [in contrast to after].' であった is the literary/written past form of だ (modern speech: だった), giving the sentence a formal narrative tone, the kind you'd find in a biography or formal account. ブラウン家 ('the Brown family / household') uses 家 (read け in this context) as a suffix designating a family line — common with both Japanese and Western surnames in this construction (徳川家 'the Tokugawa house,' ケネディ家 'the Kennedys').