@mystery is right that this is American English, but in fact there is nothing wrong with the adjective 'elder' in this situation. It is acceptable to use 'elder' in the context of sibling relationships in any variant of the language. The choice between 'elder sister' or 'older sister' is not a regional difference, as @mystery suggests. 'Elder' is still widely used in both Commonwealth and American English - it is simply a little more old-fashioned and formal than 'older'. Younger people might not use it much casual conversation, but it is fine in written language. As this is a piece of creative writing, it's entirely appropriate to use 'elder'.

Perfect! But in America we use "older sister" instead of "elder sister" :)