Hi can a 8 year old boy be motzei his sister (13 years old) in shabbos kiddish? Thank you
A child cannot be motzei his adult sibling or any other adult. The reason is because there is a rule that only a person who is obligated to do a certain mitzva has the rule of “arvus” (meaning that we are obligated to make sure that everyone else performs the mitzva). My obligation to make sure that my friend performs the mitzva is in a way like I am obligated in his mitzva. Therefore, I can be motzei him, with his obligation. However a person who isn’t obligated in the mitzva, does not have the obligation to make sure his friend does it either, therefore he can be motzei him.