I assume your parents are practicing Muslims? There is a dua the Prophet (saw) often said asking the Almighty to separate the truth from the falsehood.
Perhaps you could get them to memorize and use it, including your sister. This is by far the best protection.
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Otherwise, idk. You'd have to keep them away from the media. The powers that be have sunk billions into the art of manipulating people and they know how to pull your strings. Perhaps you've heard of Edwards Bernays, the father of propaganda? Many today are out and out deceived.
I guess its like religion. You and me know there is no truth other then Islam, however almost 5 billion people would disagree.
So why don't they see what we see? Indoctrination, bias, deceived, ect, ect, ect. Allahu alam.
There are people who know the truth and there are those who will fight it as if their life depended on it. In the end God has absolute control over every atom so seek his help in this age of deception.