Resident Return Visa - Personal Ties Requirements

I am an Australian permanent resident - should be a citizen within a couple of months - and have lived in Australia for five years. At the time I applied for permanent residence, I successfully included my two university-age children on my application. The older, after graduating from university overseas, has now moved permanently to Australia. The younger will soon graduate, but at the moment is likely to remain overseas due to his employment there. He has of course visited Australia various times during his period of permanent residence, but likely won't have accumulated two years in the country by the time he reaches the end of the five-year travel facility on his initial permanent residence visa. So, we are starting to look at what may be required in order to keep his eligibility to obtain a Resident Return Visa to eventually live and work here.

I have done some reading online but can't seem to find a clear answer as to what "Personal Ties" one must demonstrate in order to qualify for a one-year Resident Return Visa. Would simply having a father who is a citizen and a sister who is a permanent resident be sufficient? And, can one continue to apply for one-year RRV's until their circumstances allow them to take up effective permanent residence?

Thank you for your advice.