The key point is that you have to make the consulate staff to believe that you & your mom have sufficient fund to support your US journey and also have no intention to stay & not leaving US. i.e., You have a strong tie in Hong Kong!!
If you are working in a large firm or a civil servant or a professional. It would be much simple and easier. Usually a company support letter is OK. If your job might not be able to support you & your mom's journey, I think your dad's financial support / bank statement and etc. would help.
You may refer to post at another travel forum on your question.
check this out
no need to worry about the communication problem between your mother and the Officer. They have translator inthe Consulate office
about the question that they may ask is depands on your travel purpose. eg, how long willu stay in US? any relatives?very simple questions, some of the Officers are very nice, no need to be nervous, sometimes they make jokes to you. veyr nice