Pippa was an adult, wasn't she? It's true that the BRF use lots of young attendants, but royals in the past have certainly had adult bridesmaids, if only to control the little ones! The Queen had eight bridesmaids for example, headed by Princess Margaret and Alexandra of Kent, lots of cousins and daily connections, several of them no longer children.
Diana had masses of youngsters to choose from, relatives from the Spencer side and all the children at the nursery school she had worked at, for instance. Her sisters were both married, so wouldn't have made suitable bridesmaids anyway.
I guess it would depend on the bride if she wanted sisters or cousins to be attendants, whether adult or not, though there are bound to be page boys and flower girls there as well.