My girls are 4 years apart so, no, I do not think that bot needed to be included in the invitation. However, many, many times I would take the older to a party only to find that my younger was one of the only siblings not invited. There were several there her age (and we all went to the same church). Of course they had been going to the church for a long time and we were relative newcomers, but I could not believe how rude this was. (And it happened several times)