。
She really takes after her mother.
Literal
She [topic-は] very mother [to-に] resembling-is.
似る is one of those verbs that's almost always used in its ~ている form: 似ている. The action of 'becoming similar' happened long ago, so we describe the resulting state — 'is in a state of resembling.' に marks the standard of the resemblance ('resembles whom'). 母親 is a slightly more formal or descriptive word for 'mother' than お母さん, often used when discussing rather than addressing.