Mrs.sheryl, you can apply for Dependant Visa for your mother at the Immigration Department of Hong Kong, when the visa is given to you, you can mail the Hk dependant visa and her passport to your mother.

Dependant Visa 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:
1. Your mother's passport/travel document with blank pages and a validity of at least 6 months.

2. One completed Hk application form affixed with one passport-sized photo.

3. One copy of the your HK ID and your telephone number;

4. Proofs of relationship between your mother and you.