Learn Spanish (Latin America): Tus sobrinos son los que viajarán con mi yerno. - Your nephews are the ones who will travel with my son-in-law.

In this sentence, los que means "the ones who". It is used to avoid repeating the noun, so we don't have to say tus sobrinos son los sobrinos que viajarán. We use the masculine plural article los because it refers back to sobrinos.

Could los que be replaced with quienes in this sentence?

Yes! You could say son quienes viajarán (are who will travel). However, using el/la/los/las que is incredibly common in spoken Spanish when you want to point out specific people or things out of a larger group ("the ones who").

Does tus sobrinos mean only "your nephews", or can it include nieces?

It can include nieces! In Spanish, masculine plural nouns are used to refer to mixed-gender groups. So tus sobrinos can mean a group of just boys (nephews) or a mixed group (nieces and nephews). If the group were entirely female, you would use tus sobrinas.

Why does the verb viajarán have an accent mark?

viajarán is the future tense of viajar (they will travel). In the Spanish future tense, all endings except the nosotros form carry a written accent mark (like viajaré, viajarás, viajará). This tells you to put the spoken stress on that final syllable.