My big cat raised the two younger ones and today they are fully-grown and they eat and play together, groom each other, and quite frequently sleep together. Maybe it's how we treat them. The home should be a place of peace and security. When families have pet behavior problems they feel frustration and chaos. We are complex, and in some ways, contradictory.
Cats are no less so.
But there are some fundamentals. For example a basically social species like humans or dogs will instinctively cry out for help when injured. By reflex, other members of their group will come to aid the fallen member. But not with cats. When hurt, they hide out and try to recover alone because, by their nature, they are solitary.