I don't know what the magic formula is to be honest, but all I know is that I am extremely lucky that my ds 17 and dd 14 are so very close.
I don't know if its because of being pigeon pair, or the fact that I have always treated them equally... there is none of this, oh but you must do x because you are the boy etc.. if there's stuff needing done they both do it.
Although there are arguments from time to time, I really could count on one hand any times that I had to intervene, and always they just made up and carried on.
Now dd's best friend is a middle child with an older sister and a younger brother. I have never seen kids fight and argue like that. I don't know if it that is normal, as it certainly is not in our house, but for one sister to call the other a bitch, to f off our my room and that sort of thing, is just not on for me.
Rule no. one for us is to speak to others as you would like to be spoken to, even if during a heated discussion. I also reprimanded the dc when they were younger for calling someone stupid, I cannot tolerate that word as name calling, as it is degrading and you never know how the other person would take it. But, I do think personalities have a lot to do with it. Ds is such a calm, gentle kid, takes a lot to get him riled up, but when he does, you know you have crossed the line with him. Dd understands him with his ADD and tactile issues and never uses that against him. Although she is younger, ds will talk to dd about school, girls and anything really, he claims that because she is a girl, she will see it from a girls perspective as well as his, being her brother
.
So maybe its just the understanding of each other that forms the bond?