For many people "friendship" is subsumed into being "siblings"
Different people and different cultures will view this differently, but for most people "sibling" and "friend" are are at odds with each other. This doesn't mean siblings cannot be friends or friendly, but that it is a different type of relationship into which friendship is subsumed. My sister and I are now on friendly, but I would never introduce her as my friend. Our friendship is subsumed into the fact we are siblings. My acquaintances would find it bizarre if I ever introduced her as "my friend" instead of my sister.
At least in the circles I run in, it is normally assumed that siblings are at least somewhat friendly and that does not need to be specifically called out. You differentiate siblings you are not friendly with by saying something like they are estranged or distant. If you want to emphasize a deep friendship along with the sibling bond you would say that they are "close" or imply it by pointing out how much they do together.
I am obviously partially agreeing with user137369. By switching back and forth, you are conflating different hierarchies of relationship, but I'm going farther. At least in the regions I mostly deal with, friendship is completely subsumed into a sibling relationship and it would seem odd to ever refer to your sibling as a friend except perhaps in the context of also mentioning that they are your sibling and your friend, and even then it would be probably be in an informal and "cutsie" way like "this is my sister and BFF" rather than a bland statement to be taken literally.
I think this is distinct from other types of relationship where one is not subsumed into the other. It so happens I am a lawyer and on occasion I have done work for my family. Under certain circumstances my family members introduce me as their lawyer specifically rather than by the family relationship. This has yet to strike anyone as weird because the lawyer-client relationship is distinct from the familial relationship. The lawyer-client relationship is not subsumed by the familial relationship. But friendship normally is and it at least in some regions it would be weird to refer to a family member as a friend without at least also mentioning the family relationship.
How to handle this in writing
Again, this may be cultural or regional, but in writing, I would let the family relationship predominate. I would show that they are also friends by showing it through shared activities and other friendly relationships that you might not have in siblings that have grown apart. If you really want to establish it quickly and directly instead of saying they are also friends I would say something like "they were sisters and very close".