。
I'm worried about mother's health.
Literal
Mother [gen-の] body [gen-の] matter [subject-が] worry is.
体のこと literally 'the matter of [her] body' is the idiomatic way to say 'her physical condition' or 'her health' — Japanese often hedges through ~のこと ('the matter of X') rather than naming the state directly, a softer and more natural phrasing. 心配 is a な-adjective taking が for its subject of concern.