That actually happened 3 times during my interview circuit last year, and I definitely know of it happening this year as well for others on the interview circuit.
It really, really depends on (1) the professor and (2) the RA. In my case, at school #1, it was widely known that the professor hires RAs that she knows full well will become a graduate student for her. At school #2, the professor straight up told the RA that she needed to prove that she was going to learn something new and contribute beyond RA duties before she would consider her (the RA ended up going to a different program). At school #3, the RA had worked so closely with the professor that she ended up going to that program as well (though the professor didn't necessarily view her as a graduate student when he hired her).
As you can see, things really vary from school to school, from professor to professor, and from RA to RA. Even if the RA is there, it doesn't necessarily mean he/she wants to stay there. Alternatively, just because someone is an RA for a professor, it doesn't necessarily mean the professor wants them to remain there as a graduate student. In my cases, I knew full well how each professor felt about each RA (and word travels quickly), but in the end, you really just have to try your best and put all of that information behind you. You never know what the professor is really thinking, if the RA is even really interested in staying, what the RA's actual experience is, etc.
Best of luck!