The longer the indoctrination period, the longer a recovering survivor must work at recovery, and the less likely family members outside the cult organization can achieve any beneficial impact.
Brain washing or indoctrination can last a life time. It takes someone with the strength and ability to change attitudes and look outside the box and weigh up their beliefs and faith against what is the reality and what are the genuine facts from creditable sources. Very few who are brainwashed from childhood would be able to do this.
Either way, the brain has a thing called ‘plasticity’, meaning that it can add to, create, ignore, or bypass neural networks with time (and experience, thought). One thing is for sure in all cases, thought: it is harder to ‘forget’ something than to remember it. Can you imagine trying to ‘un-learn’ how to ride a bicycle?
And, unfortunately, a few conniving critically-thinking humans have learned to exploit that to dominate other humans, and do their all to prevent as much of humanity as possible to learn/switch to an autonomous (critical) thought that allows us to control, ourselves, our brain’s contents (plasticity).
You don’t need to be an expert, but you do need to know your enemy. In this case, the blocks and walls that have been placed in his or her mind, the ways they’ve been taught to mentally fight back against reason, etc. And be as kind as possible, though firm. It is a very hard thing to find out you’ve been brainwashed to any extent. And it takes a long, long time to deprogram. I don’t how long yet. Working on it though.