Parents and spouses are typically adults. Adults are can and should adhere to higher standards of behavior than children. It is expected that all children do bad things sometimes, as part of learning to not do them.

In most cases the power asymmetry is reduced compared to a parent. A sibling doesn’t typically have control of food, shelter, utilities, external relationships. A child is totally dependent on their parents for all life. Adults are generally physically larger and stronger than children, so they have a physical advantage.

So in cases of abuse by adults, the culpability is easier to discern. Its also considered that the character one has is more coherent and consistent through adulthood than between childhood and adulthood. It makes more sense to hold a 55 year old accountable for things they did at age 35, than to hold a 12 year old accountable for things done at age 5.

It is also expected that responsible adults should observe the interactions of children to detect and intervene on abusive dynamics. Even if harm came at the hands of another child, the question arises what was the involvement of nearby adults.

Sibling abuse is also often a product of parental neglect or poor parenting, which pushes the blame more towards the parents then the perpetrator. That’s not to say that the perpetrator is blameless however.

My child barely out of diapers was abused by their cousin, 2-3 years older. What adults do to children and convince them not to tell through bribes and/or shame, their brains can’t process. Adults can’t even face it and completely process it, they were physically and/or emotionally abandoned as children, don’t know how to cope, either can’t access or won’t stick with therapy long enough to to go through the excruciating part, so self-abandon, through drugs, alcohol, TV, overwork, dissociation, overeating, undereating, over and under-exercise and endless scrolling.

Or maybe the child brain doesn’t light up in certain areas and other regions aren’t developed to compensate, for various reasons.

So the answer will vary, depending on who you are questioning, for the “overlooking.” Societal and familial psychodynamics certainly play a part, but also systemic social structure and support plays an equally important part. And since society is structured to give the very wealthy disproportionately large advantages and everyone else disproportionately large disadvantages, that’s why social structures exist as they do.

What kind of abuse are you talking about? I think some kind of rivalry is normal between siblings, but then again please clarify what you mean with abuse. I think most abusive siblings stem from an a parental abuse

Why do you think so

I think that unlike with your parents, you can eliminate contact with your siblings without any consequences when you get older. It still doesn’t erase the damage, it’s just easier to forget it