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 is His slave and Messenger.
It is a fact that jinn exist, and it is also true that they affect the person by possessing him and the like. However, it should be noted that the plot of the devil is weak. Therefore, one can get rid of him and his harm by reciting the Quran, mentioning Allaah and refraining from committing sins, as we clarified in Fataawa 88599 and 86197.
If you predominantly think that your sister is possessed by jinn, then you should perform Ruqyah on her. If she is conscious about what she does, she must be advised to fear Allaah and cut her relationship with that man. She should be reminded that such disobedience may help the devil in controlling and influencing her.
Moreover, her father should be strict with her and prevent her –according to his ability –from going out with that man, as he is responsible for her in the Sight of Allaah. If it is possible to marry her off to that man [if he is religious and has a good moral conduct], then this is better as the best solution for those who love each other is marriage.
As regards what the elder sister should do to please her parents and avoid the harm of her younger sister, she may prevent her from visiting her in her home as long as she is in that situation so that the children would not be influenced by her. But she should keep in touch with her, like speaking to her on the phone and so forth in an attempt to reform her.