Sibling Rivalry Starting Too Early

Last night we went out to dinner at my Aunt and Uncle's house. My aunt asked my son R if he loves his sisters, and he said, "I like H but I don't like Z". When my aunt asked him why he doesn't like Z, he said "because she's always with Mama."

My aunt figured out in 5 minutes what I have been trying to figure out for months, why R is partial to H. I thought it was because R and H are both outgoing and like the same foods, colors, and activities. H gets his jokes while Z is still trying to figure out why everyone is laughing. Z is a littly shy too which doesn't help when we are socializing with other families with kids. Z will cling to me while R and H mingle.

I think the reason R thinks Z is "always with Mama" is because whenever there is a fight amongst them, R always takes H's side regardless and Z is left to fend for herself so I, being the "Mama", pick up Z to wipe her tears and yell at the others to stop being mean to their sister. R sees me and Z as a team and him and H as a team. Although H, being a twin, can't bear to see her sister cry and tries to console her too, but R pulls her arm and says "No H! You're on my side!"

But that's not all. I later asked R why he doesn't love Z. I said "She's your sister, she loves you," to which he replies "no, she hit me in my boys (we taught him to call his privates "boys") with my Power Ranger gun, it hurt me for days, I don't love her, she's not my sister." I'm shocked. Just because little Z, who is only 20 months old, accidentally hit her 3 year old brother in the boys with a gun, he hates her now. Why?

I gave up trying to force a resolution after I realized the parties involved aren't that capable of understanding my request. I'm hoping it's just a phase that will pass because many times I see them all playing together quite nicely, and God help anyone who happens to hurt Z outside the family. When we were at a dinner party once and some 2 year old hit Z, R went after him relentlessly and made him apologize to her for it (after wrestling him to the ground). But when a fight breaks out amongst the sibs, it's usually between R and Z, leaving H as the Switzerland in the middle.