So I wasn't a pr when they first got the visaIf your parents had strong application profiles then IRCC would have also granted a TRV to your sister when she applied with your parents in 2012 or in 2015. Your sister is the family tie for your parents to return to. This is a fairly common strategy in cases where IRCC believes the parent(s) might overstay/not return to their home country.
Also, you probably got your PR in 2014-15, and that's why IRCC granted your parents TRVs in 2015. They were again recently granted TRVs because they had already proved that they were genuine short-stay visitors on their previous TRVs.
One size does not fit all. Other applicants probably have stronger ties to their home country than your sister. Visas are not granted purely based on luck, and nothing is unfair although it might seem like that to applicants who are refused.
The first step is to acknowledge that your sister has a weak application profile + her previous applications did not work. If the documents/evidence were adequate or if they had the ability to convince the visa officers that your sister is a genuine visitor then she would not have been refused three times.
If one keeps an open mind that is receptive to suggestions then it is possible to prepare a better application
Yeah I do acknowledge that.
Now my question is should I apply again right away?
The only additions I can think of are an even more detailed explanation letter and a an affidavit swearing she will return.
Any other suggestions?