: How to refer to siblings who are friends? Two of the MCs in the novel I'm working on are half-sisters, and also great friends. Currently, when referring to them, I alternate between using "sisters"

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Family relationship takes precedence.

To emphasise that they get along well, mention early on that they are 'close'. That will set the expectation of their relationship in future and 'friendship' need never be specifically mentioned.

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Two of the MCs in the novel I'm working on are half-sisters, and also great friends.

One is an objective fact, the other is an interpretation of their relationship. As an omniscient narrator, you should stick to the former and let the characters and their actions convince the reader of the latter.

In general, if you introduce someone as your friend, people are going to assume you're acquainted but not related or romantically involved. Calling your wife or sister a friend is more than just defining your relationship in absolute terms - it's making a statement on it and letting the other person know your thoughts on it. That's not the job of the narrator, that's the job of the characters.

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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 terms, 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 "cutesie" 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 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".

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The answer you have accepted is incomplete.

Take three people:
Dan - Captain of a ship.
Adele - Communications officer on Dan's ship, and a very good friend of Dan.
Bob - Captain of another ship.

When Dan and Adele interact, you might want to emphasize their friendship and refer to Adele's friend Dan.
When they are on the bridge of the ship working, then it would be appropriate to refer to Dan's communications officer Adele.
When Dan talks to Bob in their official capacities, it would be appropriate for Dan to refer to Adele as his communications officer.
When Dan talks to Bob over a beer in a bar somewhere, he might discuss his friend Adele.

Randomly switching will confuse and irritate your readers. Switching to appropriate words in different contexts can help your reader understand the relationships.

In your example, you would refer to them as siblings when they interact with other family members. When outsiders refer to them, then those siblings could be discussed as though they are friends - that underscores that the outsiders know the two siblings, but not well enough to know that they are siblings.

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You’re using unequal terms interchangeably.

Imagine Alice is Bob’s sister and also does his taxes. The former is a personal relationship and the latter is professional; for most people those belong in distinct hierarchical levels, with family at the top.

While referring to Alice, if you kept switching between calling her Bob’s sister or Bob’s accountant, it would seem you were downgrading and upgrading their relationship all the time, making it unclear what they mean to each other. That would be even more problematic if the description were to not match the action. For example, Bob sacrificing his life for his accountant Alice is far less relatable than him sacrificing himself for his sister.

While two people being both siblings and friends is neither confusing nor uncommon, it is common to consider those as different levels of personal relationships. Whatever you feel is the most important (friendship or family) to your story and characters, stick with it once it’s established. If you change the term again, that should reflect a change in the characters’ relationship (i.e. be meaningful to the narrative).

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