Well, a few years ago - about 8-9 yrs back - when I'd tried for JJ, the scene basically was that the a lot of the seats were reserved for some or the other category. So that left only a few seats for the open category. I guess that's what someone meant by the 'Marathi Manoos' statement. i.e. If you happen to fall into any of the reserved categories, then it'll be much easier.
Also, be very serious about the entrance test, because of the reason stated above. There is a particular format for the test, i.e. still life, then something else etc. [Can't remember exactly]. So be sure to practice accordingly. In fact, people are known to take classes specially for the test. & generally, folks practice for a couple of years before attempting the test. IIRC, there's a restriction on when you can make the next attempt, should you not clear the 1st time around. So try to get info on that.
Also, someone mentioned entry through malafide means. There were murmurs that the entire process is just an eyewash & that the results are rigged, even back then. One can't really say whether there's any truth to this, cos of the quota system. i.e. I saw people whose skill level was just about average get through, while others with above average skills didn't make it. But whether those lucky few were picked cos of some quota can't be known. So try and speak to some students there and see what their take is, if possible.
And has your sister decided between fine arts, & commercial arts, architecture, sculpting etc.? She'll have to pick one at the outset.
Of course, all I've blabbed about is what was the case around 8-9 years back. I don't know whether the same holds true now. So just try and get the info afresh & only then discuss things with your sis.
All the best!