The video does not show Trump's family watching the riot. It shows them talking during the run-up to Trump's speech.
Following the upload of the video, which was featured on Donald Trump Jr.'s Instagram page, several tweets falsely claimed that the video showed Trump's family talking and joking during the riots themselves. However, Reuters, the New York Times, and the Washington Post have all noted that the timestamps in the video match the time leading up to Trump's speech, and do not correlate to the timings of the riot itself.
While the video does show Trump's family celebrating on the night of the riots, it is out of context. There is no evidence to show that Trump's family were cheering on a live-stream of the riot.