ustedes
/usˈtedes/ · A1 · pron
you (plural), you guys, y'all, yous, yinz, ye
ustedes = usted + es: the polite you, made plural
Usted is a worn-down form of vuestra merced (your mercy/your grace). Add the plural ending -es and you get ustedes: your graces, i.e. all of you. That origin is why it takes third-person verbs: ustedes son, not sois.