It seems like you're relatively low risk. Japan has several hundred cases in a population of 126 million.

It's still good to be cautious though. The negatives: air travel could be disrupted while you're here, there's asymptomatic transmission, you need to pass through a busy airport in a country with unknown infection routes, and (most importantly) the virus is much more dangerous for your parents. If I was in your shoes, I'd probably cancel and plan a trip to see everyone in a few months.