Need to know if I need to invite the family of my best man to wedding.

[maga_peter]

I am going to get married and was told by my fiance that my best man's family needs to be invited because it is tradition. I really do not know them like that. He is not married nor has kids and he still lives with his parents, so we would be inviting his mom and dad and brothers and sisters. She explained to me that it's a way to say thank you to his family for allowing him to be part of the special day. Is this true or is it just more wedding tradition myths. Please help. His family would consist of 5 more plates!