I'm no judge nor juror - but it is pretty clear that both the husband and wife had some sort of disastrous relationship. If the mother went to confront the father, then it can't be said that she deliberately decided to expose her child to a sexual act but rather that was the unfortunate consequence of her actions - which would have entailed entering the private premises of another person without their knowledge. Break and enter. Bad judgement call and her motivation seems to be anger. Women or men use their children during custody, separation and divorce arguments constantly - and that includes putting the kid front and center, sometimes as a method of trying to force the other party to visually see their obligation and responsibility.
I don't see the link between an older brother molesting the 5 year old and the 5 year old witnessing the event - unless all of the children were living in the house while the parents continued on some abusive pattern in their relationsip which affected the children. This speaks to the parenting issues of both mother and father including the issue of control, power and trust - the elder sibling using control and power to abuse the younger and the younger unable to trust the adults enough to tell them of the abuse.
A family of 5 kids up the street from me growing up, watched as their father chased their mother from the front door across the lawn of a neighbors house, where he tackled her to the ground and stabbed her 27 times. She died. They were separated at the time. They witnessed a horrible event and suffered multiple losses in their life as their mother died, their father went to prison and they were farmed out to relatives. They were forced to acknowledge, accept and then choose how they were going to let that moment in time define them - life is more than one moment - it is many.
In this case the mother was wrong. The father was wrong. The older sibling was wrong. The younger one struggling, needs to realize that you can't change what was but you can change how you allow it to define you. The only control he has is in himself - . Hope he is able to move on ...sammieswife