Brides Maids...

My step son will be marrying soon. His grandmother would like for his cousin to be a brides maid. The gm wishes to have her in the wedding as gratitude to the aunt. What is normal (proper)practice when asking bidesmaids, jr`s and so on? Should the grooms family feel that it shouldn`t be a problem since it`s normal and proper to have the groooms female family included?"She will be having 8 maids ". Thanks for all the help I can get ..bless all hp