The law is how much older t he child is from the one he shows his parts to. Six years is the standard. And we don't know if it's one time. Since I lived through it, I think it's better to be safe than sorry. There is a need to know if this was one time or often and if this is all he did or if he did more. Kids tend to be slow to tell. Been there/done that. Find out the facts and then make a decision.
This is not a hysterical mother or a hysterical system.
Facts: An eight year old boy did not agree, with his eight year old friend, to "Show me yours and I'll show you mine." That is normal and consenual.
This is an eight year old boy who made his three year old sister, who was unwilling, to let him kiss her privates then he put his penis in her face. Normal playing around? I disagree. Normal is the example above, not this huge age gap and the degree of touching an unwilling sister. One time? Maybe. It should be vigilantly explored because it could be actually more than one time and this eight year old could have a serious problem.
As one who knows the horrors of kids facing sexual abuse from older kids, I don't think it's frivalous at all and I am puzzled that it can be passed off as such and even made light of. I hope it never happens to your children, but if it does, chances are you will not be as lighthearted about it, especially if you find out it is not just a one time experience. Picture a twelve year old doing this to your little J, Malika. Now maybe the mores are different in Europe, but I'm glad we explore things carefully here because sexual abuse is a trauma that children do not forget. This child is not playing with his peer. He is obviously forcing his sister, who is a baby, to let him do things to her. It may be the only time he did it, which is good...then he gets help sooner. But it could be that he has done it many more times. And certainly nobody wants this to become a ritual. Every sexual abuser has a first time. An innocent romp about sex is with same age peers playing doctor. Six years is a big age gap here.
Having said all this, I do think they should find out the whole picture before the oldest child is taken to an Residential Treatment Center (RTC). But the bigger boy is now going to have to be watched whenever he is with his little sister, even if he stays home.