Siblings, not just for foot rests!

I have a little brother. He's not so little anymore, at 6ft, and 250lbs about, married with his own 2 kids. I'm really impressed that he survived. Or that either of us did. It gives me hope for my two munchkins.
Throughout the years, we put beatings on each other that you wouldn't believe. Or maybe you would, if you have siblings.
I once stabbed him in the eye with a McDonald's straw, he had a blood red half-moon in his eye for about a month. He once slammed my fingers in the car door and then locked it, preventing my escape. He got stitches on an easter morning, following a rock fight involving me and all our cousins. He likes to point out that we had the advantage of starting at the top of the hill, I think it's just more evidence of me being at the top of the food chain.
And here, in my very own home, it begins once more.

I thought things would be different. In my family, Tyler is the older brother, and Kendyl is by no means a tomboy. I try to impress upon them that Tyler's job as the big brother is to protect and care for his baby sister. To tolerate and spoil her. Apparently, in 3 year old, that means to torture, push her around and make her scream. In his defense, her favorite activity is to be as annoying to him as humanly possible.
When I was a child, I clearly remember seeing my brother as a total usurper of my childhood. All the good stuff went to him, he got away with murder, he was an idiot who messed with my stuff and didn't know how to wipe the snot off of his face. And all that held true throughout our teenagehood.... except for the snot, mostly.
To be fair, I think he saw me as a bully, who told him what to do at every turn and tattled at the least infraction. But he got up extra early to eat the last of the sugar cereal in a giant mixing bowl so I couldn't have any, so he got what he deserved. Plus, all that sugar cereal made him fat, so that's a bonus.
As adults, I now love and appreciate my brother. We still pick on each other, in a more subtle and adult sibling way, but we can be friends now. Probably because he doesn't come over and eat all the sugar cereal, although, last time I did laundry at my parent's house, he had gotten to the chocolate grasshoppers first... bastard.
Well, my brother's and my relationship now gives me hope for my children, although if I have to put up with 18 years of rock-throwing and eye-stabbing I might lose my mind.
Right now, my whole day is taken up with breaking up pushing, hitting, grabbing, screaming, pinching, fingers in drawers, fighting over things that aren't supposed to be played with anyways, interfering with each other in creative ways like messing up neat lines of cars, aquisitioning both sippy cups, getting too close to each other's faces and the occasional rear naked choke hold.
They are 20 months apart. I'm now thinking I was totally insane for getting myself into this mess to begin with. When he was little (God, what a dumb phrase to describe a 3 year old) he was so easy, I thought, double the trouble? I can handle that!

No one told me the trouble doesn't double, it cubes. Trouble to the second power. Because not only do I deal with him and then her, I now deal with him, her, him dealing with her, her dealing with him and the trouble both of them can get into when they decide to gang up on me. Seems to me that this is valuable information that SOMEONE might have mentioned before I got into this mess.
But I get stuck with the only Italian mother on the face of the earth who doesn't give uninvited advice. Anyone who has one of these nosy mothers or mother-in-laws, be grateful! Alot of good information comes from these people. I understand that it might be annoying, mom all up in your grill and all that jazz. But at least you don't have to pry advice or life lessons from these women like prying barnacles off a boat.
The way these kids just automatically deal with each other astounds me. It would be one thing if they just hated each other. Yeah, bum deal, but at least I would know what always to expect. But they like to throw me a curveball every once in a while. For instance, I can not give Tyler snacks without him dividing up his spoils and giving half to his sister, or at least pointing out that she needs her share. And, sometimes, instead of destroying carefully lined up rows of Hot Wheels like a diaper clad Godzilla attacking Tokyo, I've caught her collecting all the cars strewn around the room and bringing them to him to line up... and him ACCEPTING them, gracefully, even when her uncontrolled running back and forth upsets his orderly rows.

And something that I'm wary of, because I can just see someone taking a header into the dresser... the nightly ritual of running back and forth across the house screaming laughter together. I'm loathe to break it up because it makes them both so happy. Eventually it will end in tears, but for right now, they are playing happily together. Who am I to stop it?
I think I realize, that, just as my parents had very little to do with the relationship between my brother and I. I have very little say in the relationship building between my two little ones. It's kinda their thing, and although I can help minimize the bloodshed by playing referee, or can give problem solving advice to help them play more cooperatively, the relationship is theirs, and will always be theirs, not mine.
This post seems a little open-ended, and my past English teachers would be rolling in their tenures (not graves, dummy, I'm not that old) if they read my story lacking a conclusion... but like I said, this is my kids' story to complete, not my own. Hopefully it turns out well.