Its actually not that hard to change the status from b2 to f2..my wife has the same problem too..you need to have all the necessary documents needed, you can get that list from the school..feel out a form from the uscis website or it can be obtained from school also. send it to uscis head office along with all the documenst and a cheque for three hunderd something dollars..it takes about 3-4 months to get approved..is your sister going to be your dependant?? if thats the case then i am not quite sure if that would work..if her husband is here on f1 then its a sure shot..