、。
The mother is looking after his care.
Literal
Mother [subject-が], his nursing [at-に] is-attending-[ongoing-てる]
看護にあたる = 'to attend to nursing/care' — あたる (to hit/face) takes に to mean 'to take on / deal with.' あたってる is the casual contraction of あたっている (い dropped). The comma after が sets off the subject.