I was reading how Demonic spirits ae the cause of it. It's in a book i was reading, very amazing.
I recommend you pray for Deliverance for your sister and that Satan's hold over her and any demonic bondage be broken and that she be cleansed by the BLOOD of our Lord Jesus Christ.
Pray to God to reveal to you if she needs deliverance, and get back to me via private message and i'll show you the simple steps to carrying out an Exorcism
As Christians we have Gods authority and the power to cast out spirits
Kind Regards,
Scorpz.