They still have a girls and boys school at the old Roloff campus in Corpus Christi, TX (where I was raised and still have family). Here is their official link:
http://www.pbc-roloffhomes.org/content/Here is their contact information: 1355 old brownsville rd | Corpus Christi, TX | 78415 | 361-852-1155
You are able to go to church services on Sunday and if your sister is there she will be in the congregation. The Roloff Ranch (boys) in Enterprise Alabama has been closed as of January of this year and the caretaker was arrested and is waiting trial. I live in Alabama and it made all the news stations. I think the girls dorms are still open but they are under investigation.
Everyone has differing opinions about whether these schools are for the good or ill. IMHO their methods are over the top. I was at Bethesda. I look at it as a possitive experience that changed my life for the better - BUT that is because my home life was sooooo much worse than being in Bethesda and you can learn things from any situation. I chose to hang onto the good and let go of the bad (please don't slam me for that comment). I would NEVER advocate their methods as a possitive parenting model. I just chose to make lemons into lemon-aide and made a conscience choice to think of the possitive outcomes in my life because I was taken from a much worse place, my home.
If your sister is in either location (CCTX or Enterprise, AL) it should be a fairly easy thing to discover if you have the money to go to these locations in person. The authorities in both cities may even help you determine if your sister is being held against her will by opening or reporting on an open investigation if you call them. I know the local Sherrif's office in Neuces County has been known to be pretty helpful for some.
Good luck. Family should NEVER be forceably seperated and nobody should be left wondering if there is anyone out there that cares about them. Keep looking for your sister, she will appreciate it when you find her.