。
She's the one supporting the family.
Literal
She [exhaustive-が] family [object-を] is-supporting.
Note 彼女が, not 彼女は — the が here is exhaustive: 'it's specifically she who supports the family,' picking her out from possible alternatives. With は, 彼女は一家を支えている would just describe her ongoing role; with が, the sentence answers an implicit 'who?' 一家 ('whole family, household') is more compact and more formal than the everyday 家族; it often appears in idiomatic frames like 一家を支える ('support the household') and 一家団欒 ('family gathering').