Learn Spanish (Latin America): Mi cuñada y mi sobrina son las que prepararán la fiesta. - My sister-in-law and my niece are the ones who will prepare the party.

In Spanish, you can combine a definite article (el, la, los, las) with que to avoid repeating a noun. This combination translates to "the one(s) who" or "the one(s) that". Here, las que refers back to the women mentioned earlier, acting as a shortcut so you don't have to say "the women who".

Why is it las que instead of los que?

The article must match the gender and number of the people or things it replaces. Since it refers to mi cuñada (my sister-in-law) and mi sobrina (my niece)—two females—we use the feminine plural las. If even one of the people mentioned were male (for example, a nephew), the masculine plural los que would be used instead to group them.

Could I use quienes instead of las que in this sentence?

Yes, you could say ...son quienes prepararán la fiesta. Both are perfectly correct and mean the same thing. However, using combinations like las que or los que is incredibly common in everyday spoken Spanish for expressing "the ones who."

What tense is prepararán and how is it formed here?