I'm an only child and my DS has been an only for years too.
We now live full time with my partners daughter and part time with his other 3 children.
One thing I have noticed is although none of the children really argue (I was expecting this!) they just snipe at each other. The eldest four are all teens, and the youngest is 9.
So I was expecting whinges of "she took my hairbrush/game/new top", followed by a bit of shouting etc. but what we seem to get is "oh god, you're not playing that are you, you're so sad" to DS playing a board game with me and DP. Or when I'm helping DSD with homework, "can't you even do that yet?" from her sister. Even the youngest takes great delight in trying to get me to agree that X/Y or Z is not as good as her at A/B or C. The eldest snipes at me and her father too. Putting down things like what cutlery I lay the table with to what car I drive.
There are constant nasty put downs and I really dislike it. It's not unusual for any of them to say anything nice to each in other in a day and the dinner table is just a constant round of one up man ship being batted around the table roughly disguised as "just having fun" and "teasing". It doesn't feel nice and I don't like it but having not had much personal experience of siblings, is this normal?