The police can arrest you anytime, but they can only charge you if they have evidence that you committed a crime.
It’s my understanding that the brothers were together at the time the murderer-rapist was arrested. It’s likely the other two were arrested in case they were involved.
Sounds like I could be sitting in the front yard having a glass of tea with a neighbor who, unbeknown to me, is a monster and become implicated.
I really doubt that Ariel confided in anyone. What would he say? “Hey..I got these three kidnapped girls in the house. They have been there for 10 years”.
In any case the girls..no, ladies, will at some point say if anyone else was involved. I suspect they already have or the brothers would not have been released.