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As a US citizen, you do not need a visa to visit Hong Kong SAR for a short visit of up to 90 days.
You do, however, need a visa to visit mainland China. As you are visiting family, you may be able to apply for the Q2 family visit visa, which allows you to remain for up to 180 days.
You will need to have at least a double entry visa, as when you visit HK, you are considered to have exited China and will need another entry on your visa to return to the mainland. The good news is that under a 2014 bilateral agreement, US citizens usually get multiple entry visas without difficulty, as long as your passport has at least 1 year until its expiration. Be sure you tick "multiple entry" on your application and provide your itinerary including the Hong Kong visit.