What's the adjective to the word "sibling" [closed]

I'm looking for the adjective that goes with sibling. My sentence would be something like:

The [...] relationship.

So the adjective would be used to denote that the relationship is one between siblings. A native speaker told me it was simply sibling, however, I'm not convinced. I have tried googling for the answer, but it hasn't yielded any results as of yet.

Unfortunately, my Oxford dictionary is having a sleepover at one of my fellow students.

Your native speaker is right about sibling relationship.

If you want to talk about the relationship between brothers, there's fraternal.

Conversely, sororal is used for sisters.

While it is broader (and can cover relationships outside of the same generation) the term consanguineal reflects common blood relationship

having the same ancestry or descent; related by blood.

It derives from the noun form, consanguinity, meaning

relating to or denoting people descended from the same ancestor: