Your parents can apply for Hong Kong Dependant Visa which is issued to the lineal relatives of a person who is a holder of an employment visa (except foreign domestic helpers and imported workers), or a student visa of HK. The following additional documents are required:

a. One copy of the sponsor's HK ID and the sponsor's telephone number;
b. Proofs of relationship between applicant and sponsor.

If your brother doesn't work in Hong Kong, your parents can only apply for Hong Kong visit visa for family visit, they can try to ask for 3 months stay duration.