Sister is a judge. Can she be my brother's agent ad litem?

Of course.

Article 58 of the new Civil Procedure Law stipulates: "The parties and legal representatives may entrust one or two persons as agents ad litem. The following persons may be entrusted as agents ad litem:

(1) Lawyers and grassroots legal service workers;

(2) Close relatives or staff members of the parties concerned;

(three) citizens recommended by the community, units and relevant social groups where the parties are located. "