I think it can be expected. People, especially family members, easily rub each other the wrong way. Just look at the divorce rate and how badly some adults behave, and they know better. Adults are also better at hiding their bad behavior.
My son is dealing with this annoying person (his 6 yo brother) he can't get rid of who pesters him 24/7. At least that's the way he sees it and the fighting usually only stops when they are separated.
They have NEVER got along together for extended periods of time. They are complete opposites in every way. I was just thinking this morning, they are like oil and vinegar, no - more like fire and water. It is constant, non-stop bickering, arguing, fighting. Even when they are playing it goes too far and one will get upset or hurt. When their friends upset them, or hurt them, they can take a break and go to their perspective houses. Unfortunately, siblings do not have that option. Some separation can keep the peace.
On the rare chance they are playing nicely together, I always give them plenty of praise and tell them know how nice it is and how much I appreciaete it. That way I'm not only give them attention when they are fighting.
Now that they are older, their fighting can be more physical with scratching, slapping, pushing, etc. None of this is tolerated, as I do not want this behavior spilling over into school, etc. etc. They get time outs, groundings, TV and game time taken away, whatever works for the moment.
I too was an only child and to watch these two bicker constantly, I’m glad I did not have siblings. Without being separated, my 9 yo has no peace. For any of us to have any peace, the two have to be separated or both parents present to control it. My husband and I are constantly tag teaming to keep it under control, when he’s around that is. It’s not only sad because I love taking my kids everywhere, but their constant fighting limits some activities merely to save my sanity. 50% of our outings are without Dad.
I took them on a road trip 2 years ago by myself to go visit Grandma and I’ve vowed to never do it again. The worst was driving in California, Bay Area rush hour traffic. Traffic was going about 85 MPH, cars 3 to 4 feet apart; I am not kidding. One wrong move and there’d be a pile up. I’ve driven in it before and it doesn’t bother me, as long as I can concentrate. But try concentrating with 2 kids in the back, “Mom, he’s mocking me; Mom, he hit me; Mom, he won’t give my ? back; Mom, he won’t leave me alone; Mom, he kicked me; Mom, aren’t you going to give him a spanking? MOM, MOM, MOM, MOM!!!!!
I was trying to ignore them, trying to concentrate on the traffic. One slip up, that’s all it would take. I was also trying to figure out where to go –take this exit or that? Freeways and roads had changed since I was there last. The more I ignored them, the louder and more physical things became, the screaming started. AND THEN I PULLED OVER, doled out punishment and threatened to turn around, go home, leave them with a sitter and take vacation by myself.
This always works (for awhile), because they know I’ll do it. I never threaten anything I’m not prepared to back up. I have canceled family vacations and returned earlier than planned for fighting with them screaming and crying all the way home that they will behave, even when it cost me money to do so. I have a "3 strikes and you're out" rule and I stick to it. It’s hard, but they have to learn, and they are, slowly but surely.
They are starting to remind each other on trips not to do this or that because Mom will go home or not bring them. They need to learn to get along because taking only one is not an option; they both go, or neither. Sometimes, separating them is not possible.
I’ve been told by everyone this is “normal”. Some say it gets better, others say it gets worse. Some have kids that get along and actually conspire against them; not sure what’s worse. I don’t know, but it drives me nuts. None of this probably helps you, I have no advice, as all kids and relationships have different dynamics, but I guess you’ll know you’re far from alone.