。
She is his real mother.
Literal
She [topic-は] his [genitive-の] real [genitive-の] mother is.
実の ('real, biological, genuine') is a no-adjective (modifies nouns via の) used to assert a true/blood/genuine relationship — 実の母 ('real/biological mother'), 実の子 ('biological child'), 実の話 ('a true story'). Implies a contrast with a step- or adopted version. Compare 本当の ('truly, genuine, real') which is more general. The polite です gives this a slightly testimonial tone, like a clarification or revelation.