It's the bride's prerogative entirely who she chooses to be her bridesmaids and flower-girls. In the same way the groom chooses his own best man and ushers. It's the expectations of other people who assume or are upset, by either themselves or their children not being part of the main bridal party, that cause friction in a lot of family weddings. You can never please everyone with the best will in the world. We are going to a family wedding this year were the bride has chosen three friends as her bridesmaids, all in their twenties, and not one small bridesmaid ,even though thre are several little children within the family. If your daughter wants her sisters little girl as a bridesmaid then she will no doubt ask her herself.
Recommendations please, for a stopover on the way to Loch Tay