There is no doubt that your choice should be to ask your daughter first, as you'd prefer her to be, and no doubt she would also be thrilled to be asked. If she says she'd prefer to step back and not have so much responsibility then you're left to ask your twin. You can explain to your twin that you'll need her support far more throughout your build up for your big day and arranging your hen night etc, but that as your daughter is only 12 and therefore at the age where being a bridesmaid is such a privilage that you feel it wouldn't be fair to deprive her of the honour. Your sister will understand as she was 12 once too, and will have far more fun ridiculing you on your hen night!

It depends what the title 'Chief' Bridesmaid means to the Bride & Groom. This role traditionally holds some responsibilty, such as organising the hen night, taking care of the bridal flowers and dress and also dancing with the bestman! If this is the case your sister should be given this job. If it just means that they call themselves the chief bridesmaid and nothing else then its your wedding and I am sure that you twin sister would be caring enough to understand that it is a big day for your daughter as well as you and your partner. Some churches do perform 'Family' ceremonies where children can be more involved in the service, please call me to find out more. Hope this helps.