good advice here....
you could also tell your sister that you are very concerned about her.
She is trapped in a mind control cult and you and your mother are very worried about her.
Encourage her to read an encyclopedia, visit the local library, or heaven forbid do some research on the internet. See if the religion she belongs to can stand up to some scrutiny.
JW's need to realize that they are the ones in trouble - they are trapped in an evil organisation and they have been thoroughly decieved. Do what you can to wake her up. The awakening and transformation that occurs is a beautiful thing that all JWs deserve to experience (except for the really nasty ones)
Cheers,
The Oracle