You don't get a power of attorney over someone's estate. You are either appointed by the will or by the court to serve as the estate's administrator. If your sister has already been named the administrator or executrix, then you would need to prove to the court that she is unfit to carry out those duties and that they should assign those duties to you. Age will likely be irrelevant as you're both legally adults.
As far as you being upset about them not contacting you prior to your sister being appointed, there's no remedy for that as they are not required to contact you. It is up to you to petition to become the administrator if one is not appointed by the will.
Good luck with that! I did that same sh*t 15 years ago at the University of Stellenbosch. The only good thing that came from it, was that I realized I had a knack for languages. I'm a language practitioner now and can speak 5 of the 8 languages I studied while doing my Law Studies. In SA you need to know a few languages to practice Law. As your professors might have pointed out to you. You also need to do some Accounting courses.
These were the courses I followed for the first year:
Private Law (Persons and Family)
Accounting for Law Students
I did three years of Law Studies and eventually got my degree the year after in Languages. I now speak Korean and German as well. In my second year I did more language courses, because I knew I was never gonna make it in Law. My marks were too low.