His question is not about his parents travel to Australia, his question is about if the embassy determines he provided false information during his interview by not putting his brother in family relationship certificate. Coz if the officer asks his parents why did they go to australia and they responded to visit their other son, the officer might say that son was not listed in family tree when he applied for his visa. So if the officer determines he provided false information or hid the information during his interview, his parents visa could be denied.
In my opinion, there is a risk but it depends on the officer. good luck