I thought the rule was just that the minimum was 2 witnesses and I didn't think there was any rule at all about representing either party. It's far from unusual to find that they are the same sex too, although more often two male than two female in my experience of reading through church registers.
Sometimes they just seem to be regular witnesses (maybe churchwardens or people who just like to be there because it's a wedding) and very frequently, when clearly related, they appear to belong to one of the families only. For my parents, it was his sister and husband and for my paternal grandparents, it was her brother and sister.
I think that, in recent times, it has just become traditional to have "best man" and chief "bridesmaid" as they are specifically there by invitation of the bride and groom to witness the wedding.