All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
First of all, we ask Allaah The Most High and The Most Powerful to bring your sister back and cure her from the disease which she is suffering from.
As regards informing you about where she is now, then this is something that we do not know as this is a matter of the unseen which is impossible for any creature to know.
As regards seeking the help of the jinn, it is not permissible even if this is for a permissible matter; for more benefit, please refer to Fatwa 84297.
Hence, we advise you to earnestly supplicate Allaah, especially her parents. The supplication of the parents is answered; the Prophet said: "There are three supplications that will undoubtedly be answered (by Allaah): (...and he mentioned) The supplication of a parent for his children." [Ibn Maajah]
Nonetheless, there is a special supplication for finding a lost item. The supplication is: “O Allaah! You are The One Who brings back a lost item, and The One Who guides the misguided, and You surely guide from misguidance, so bring back my lost item by Your Might and Power, as it [the lost item] is from your Bounty and Favour.”
This narration is Dha’eef (inauthentic) and not confirmed from the Prophet yet some scholars stated that it is permissible to use the wording of the supplication in it.
Therefore, you may supplicate with this supplication. You should undertake all permissible means which may help you find your sister.
It appears that she is suffering from a psychological disease or magic or is possessed by jinn. In case you find her, you should seek the appropriate cure for her; Ruqyah is beneficial in all cases.