In English there is siblings, in German Geschwister. To the best of my knowledge, French does not have a single word but rather uses (les) frères et sœurs construction. So does French lack indeed a single word as I think, for such a basic situation? Can we use sibling(s) as it is in French?
I’m a Francophone here and no we don’t have a word for "siblings" in French, actually not in Canada for sure.
By the way, I would never use "adelphe" ou "fratrie", because the word adelphe is not known in French even if it’s in the dictionary, we never use this word. For the word "fratrie", I don’t know about France, but in Canada people will probably be confused, because even me I had to search to found the real meaning of the word.
To make sure everyone understands, just say : frère et sœur, comme dans la phrase: As-tu des frères et sœurs?
NB : By the way, don’t forget that there are a lot more words in English than in French.
Well, If I refer myself to the French Academy Dictionary, the word "fratrie" would be more likely to be defined as the "group composed of your siblings" (siblings having there the emphasis put on the individuals obviously). And adelphos/adelphé are greek words and subsequently don't have an entry in the dictionary, at the very least they weren't used in French around the publication of the 1st Edition of the Dictionary (1694) until now, meaning they were never used...
source (in French): https://www.dictionnaire-academie.fr/article/DNP0109
Sibling used to mean relative, brothers-and-sisters is a modern evolution of this ancient germanic word.
Geschwister has a feminine bias (Schwester).
Fratrie has a masculine bias (frère).
All these words, included the French word contrary to your supposition, are used to mean brothers-and-sisters, but not the Greek φρατρία the meaning of which being more tribe than family (another evolution of the Indo-european root).
Single words have been found but are not countable (in context, for individual siblings), unlike frères et soeurs. This simple solution should prove insightful:
— Vous êtes combien chez vous ?
— Dans ma famille nous sommes quatre enfants, en plus de ma mère et de mon père.
A translation to siblings (or even a substantive in the question phrase) is not required for this basic situation.