All siblings fight, same gender or opposite gender.
I have a boy and girl. 4 years apart.
They fight at times.
But generally they get along like 2 peas.
BUT the thing I have taught them since they were young, was that, they are family, they are siblings, they will have each other when they are old too, to have each others back, to look out for each other and protect each other etc. They are family.
Each of my kids are different in personality. So, I have also taught them, since they were young, to know... each others cues and their own. IF one of them is not in the mood, for example, to play together and wants to just play by themselves, they can say so.. But if the other one keeps nagging the other to do something they don't want to do, then naturally anyone would get "irked" with that. So to know, the cues of the other. And respect that. ie: sometimes my son (who is the younger one) will tell his older sister to stop bothering him. He says it in a nice, tone and politely. BUT sometimes sister doesn't listen... and then keeps prodding her brother. It then becomes, IRRITATING to brother. THEN a "fight" ensues. Because, the sister did not listen nor respect, her brother saying nicely... to stop it. Using that example, I then tell my kids... "your brother TOLD you, NICELY, to stop it and back off. He's tired. You did not listen. He tried again to tell you NICELY. But you did not. THEN his patience... got to zero. And then his voice escalated... and then you both fought. BUT, if you simply stopped when he asked you to, nicely, then this fight would not have happened. You need to... listen, to when your brother says something and you know darn well, you were irking him. So don't act surprised when he then gets MAD, at you, for not heeding to his asking you to stop, in the first place." It is about, knowing the others cues... and your own.
And I always tell my kids, they are not the same, nor the same age, nor the same person. THUS, each has different ideas. And that's okay. BUT, they are brother and sister, and they need to remember that.
Even if my kids are "competitive" about certain things... I tell them that that only makes friction. Not teamwork. EACH of them, has different talents and different ideas of what is fun. They do not have to be the same or "better" than the other. They are each, themselves.
Now, speaking for myself growing up with a sibling of mine. We are close in age, and it was hell. Because, that other sibling of mine... generally was just a jealous controlling egotistical person. And thus, she was always trying to sabotage me.
If siblings have any jealousy or controlling tendencies... over the other sibling, then that is also when, problems will occur.
Sibling fighting.... is NOT always a "developmental thing."
Some siblings get along well. Some do not.
It is also per the personality and inherent attitude, they have toward others/the other sibling. And if they can come to terms with that and themselves, or not.