。
She treats me like a mother.
Literal
She [topic-は] me [to-に] mother [as-のように] does [for-me-くれる].
母親のようにする ('do like a mother') uses the simile pattern Xのようにする ('act like X / treat like X') — combining のように ('like, as') with the auxiliary する. Stacked with the benefactive ~てくれる, the sentence carries warm gratitude: she takes a maternal role with me, and that's a kindness. 母親 ('mother') vs 母 ('mom, my own mother') — 母親 is the slightly more general / formal noun, often used when describing the role rather than referring to one's own mother. With に, the speaker is the recipient of this treatment.