。
She's like a mother to me.
Literal
She [topic-は] me [for-には] mother [like-みたいな] thing is.
Several pieces working together. (1) 私には ('to me / for me') uses the composite particle には, where に marks the relevant scope and は topicalizes/contrasts it — softer than plain 私には as a statement of perspective: 'as far as I'm concerned.' (2) みたいなもの ('something like, kind of like') is the everyday simile pattern for figurative equivalence — softer and less literary than ~ようなもの. (3) 母親 is 'mother' with a slight emphasis on the parental role; お母さん is more familial. (4) です gives the sentence a polite, considered finish. The whole sentence is a warm, slightly understated declaration of closeness — not 'she IS my mother,' but 'she's something like one to me.'