Praise be to Allah.
We ask Allaah to take away from your cousin what she is experiencing and to heal her, and divert that jinni away from her, for He is All Hearing and Ever Responsive.
The remedy in this case and others is ruqyah as prescribed in sharee’ah, which may be done by the sick person himself or by one who is treating him, and it will be beneficial by Allaah’s leave. But it may need to be repeated and you may need to do other things to help it. We will mention the most important of these things here:
1 – Reminding the sick person to always recite the adhkaar for morning and evening, when going to sleep, eating, drinking, entering and leaving the house, and to say a lot of dhikr in general. The best of dhikr is reciting Qur’aan and the most important passages to recite are the Mu’awwidhaat (soorahs seeking refuge with Allaah), al-Faatihah, Soorat al-Baqarah and Aayat al-Kursiy. The more the sick person perseveres in dhikr, the more the shaytaan’s influence on him will weaken, and he may be expelled by means of ruqyah.
2 – The ruqyah should be done by a righteous person who adheres to the Sunnah and who shuns bid’ah, humbug and witchcraft.
3 – The sick person should turn to Allaah a great deal and ask Him for help, for Allaah answers the prayer of the one who is in difficulty and relieves his distress, as He says (interpretation of the meaning): “Is not He (better than your gods) Who responds to the distressed one, when he calls on Him, and Who removes the evil, and makes you inheritors of the earth, generations after generations? Is there any ilaah (god) with Allaah? Little is that you remember!” [al-Naml 27:62]. Especially if the du’aa’ is offered at times when a response is more likely, such as between the adhaan and iqaamah, just before dawn, in the last hour after ‘Asr on Friday, when breaking the fast, and when travelling.
4 – Always offering the five daily prayers on time.
5 – Repenting to Allaah from sins that may be the cause of the shaytaan gaining control of a person.
6 – Purifying the house of things that the shaytaan loves, such as images, dogs, music and singing. Imagine this devil being cornered and having no food or drink or pleasure. He may be deprived of that by saying the name of Allaah over food and drink and by getting rid of things that he loves. Would he want to stay?
And Allaah knows best.