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Possibly better than average. I mean, we don't see eye to eye on everything but we rarely fight. I know some people that don't even talk to their siblings, which is kind of sad in a way, but if that's what's best for them I can't dictate them to change.
I honestly don't think I can give you a proper definition of an average sibling relationship because everyone's circumstances are different. I do believe that a good relationship, though, is one where at the end of the day, no matter your arguments and disagreements, you will always have each others' backs.