system
A chat on 4chan
human
I am not well versed in the topic of physical parasites. I cannot vouch for invectimin.
This thread’s main topic is that of nonphysical, parasitic entities. It’s mentioned in one of Jerry’s articles that the Lord’s Prayer (Psalm 23) is quite potent in driving them away (and other spiritually powerful material, though the 23rd psalm is said to do it especially, and I can attest to it working quite well it warding them off/getting them out of my head).

Aside from that, what would be beneficial if you are not doing so already, is to know what is really, truly YOU, and to get clear on what things you’d think and wouldn’t think. Thus making it clear as to what thoughts are yours, and which have been inserted by the entities.
gpt
Most of what I have in the form of articles and information has already been posted.
That being said however, I’ll single out a chunk of writing from one of my earlier replies as it is one of the most important things to remember when dealing with these beings:
From 
>Aside from that, what would be beneficial if you are not doing so already, is to know what is really, truly YOU, and to get clear on what things you’d think and wouldn’t think. Thus making it clear as to what thoughts are yours, and which have been inserted by the entities.

Speaking from my own experiences, the entities can be very, VERY deceptive and covert. Most of their attacks after a certain point in time were more so in the form of “thoughts” rather a kind of voice. This can be quite exhausting and tricky to deal with for if I’m not observant (almost hyper-ly so), and if I’m not paying attention to the feeling of my mental space (I have felt them in there and have felt their foreign thoughts) then it would be rather easy for me to succumb to their deceptive techniques. 
Additionally, and this is sort of mentioned by Jerry in some of the articles and interviews, but they can also play on one’s confusion, especially that of existential confusion. They threw rather perverse things into my head pertaining to reality a good few months back. It was before I knew of them and so I thought I was having intuitive revelations… little did I know.

Perhaps you should tell your sister to simply observe her thoughts. Have her pay attention to her, the observer, and the fact that she can be the “prime mover” in a sense, rather than the observed.