Learn Turkish: Bu küpeler sizinki mi yoksa onlarınki mi?

The suffix -ki acts like a pronoun that replaces a noun we already know (in this case, earrings). When added to a word in the genitive (possessive) case like sizin (your) or onların (their), it creates sizinki (yours) and onlarınki (theirs). It prevents us from having to repeat the word küpeler.

Since küpeler (earrings) is plural, shouldn't it be sizinkiler?

You can add the plural suffix to make sizinkiler (those which are yours), but it is optional here. Because küpeler is already explicitly plural at the beginning of the sentence, Turkish often drops the plural marker on the later pronoun to avoid redundancy. Saying sizinki here is perfectly natural.

Why use yoksa for 'or' here instead of veya or ya da?

Yoksa is specifically used for 'or' when you are asking a question that presents two distinct alternatives (e.g., 'Is it A, or is it B?'). You will usually see veya or ya da used for 'or' in regular statements, while yoksa is the standard word for mutually exclusive choices in questions.

Why is the question particle mi used twice in this sentence?