My girls are 6 years apart and as they were both growing up, the older one bullied her younger sister, I mean not exactly what the word denotes but the younger one is secluded from being around the older sister's friends. The bedroom door is always shut when her friends are around keeping her little sister "off limits" almost at all times plus, blaming issues. There was very little attachment and caring between the two for their age gap is far from the other. Now, it is a relief for me to see them both grown to even have their own boyfriends and their communication, attention, affection including a time for play with each other has really improved a lot especially that the younger one is in her teens. My daughters- one is 22 and the other just turned 16.
I and my siblings (4 of us) have a gap of 2 years in-between. I am closer to the third (a brother) than another brother after me. Understanding that fights cannot be avoided during childhood, but changes eventually as we grow older even with the relationship with the brother after me and our youngest sister. We didn't get along well when we were kids. But each of us had our own favourite brother or sister. At the same time, love, closeness (absence that got us closer via overseas communication), the need for advice, and missing each other started to flower and marked an important role when growing up meant parting ways.
I think the perfect gap for me is 2 years.