ethnically Asian, the Chinese aren't going to take any drastic actions against an American citizen unless they are actively involved in the demonstrations some how. It would be a PR nightmare for them if they even temporarily arrested a family member of a Caucasian or African American traveling in China related to this.

At worst, they would revoke their visa and tell them they aren't allowed back in the country, but they are very PR conscious and aren't going to risk looking like the bad guys by arresting a US citizen over twitter comments. Maybe if you are a nobody with no ties to a celebrity, but with the environment right now being what it is, they want to look like they have the moral position in regards to Hong Kong, and they don't want to jeopardize that.