Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear questioner, thanks for your question and we implore Allah earnestly to rescue your sister from all evils and to guide us all to the best both in this world and in the Hereafter.
One of the basic aims of the Islamic Shari`ah is to free human beings from harmful, burdensome customs and superstitions. Ascribing a miscarriage to some local superstitions is an innovation that is unacceptable in Islam.
In her response to your question, Dr. Zeinab Al-`Alawani, instructor of fiqh and Islamic studies, Graduate School of Islamic and Social Sciences, Leesburg, Virginia, states:
First of all, I would like to remind you of Allah’s saying, “Allah knows what every female bears, and that of which the wombs fall short of completion and that in which they increase; and there is a measure with Him of everything.” (Ar-Ra`d 13:8)
Allah explains in this verse that He is the One Who knows about everything that is related to pregnancy. There are natural laws related to this process and it depends on certain medical reasons that cause the miscarriage. There is no such thing as jinn that can cause miscarriage—this is all superstition.
The idea of blaming jinn or something else is a wrong methodology in understanding the Qur’anic teachings. Your sister needs to consult a specialized doctor about her case and then ask Allah for good health and a successful pregnancy.
Allah Almighty knows best.