Struggling!!!!

I seem to have everything sorted, apart from Bridesmaid dresses!!!!!! I know what i want .....I think....but just cant find what Im looking for! My chief bridesmaid is my best friend, who is 40, but with a fab figure! I want her to wear a silver cocktail length dress, with straps...not disimilar to the shorter dresses on BHS. I then have my 2 nieces, who are 12 and 10. The 10 year old is a tomboy and has never worn a dress! Every style i choose, my sister keeps saying she wont wear it....Either she wants to be a bridesmaid and wears what i choose or she doesnt! The other niece is 12. I wanted to put her in the same dress as the older bridesmaid but my sister says its too grown up for her! i want the girls in pink knee length dresses! Actually.... i guess this is really a rant about my sister!!!!!!! Anyways, if anyone has seen any pale pink dresses that would suit a 12 and 10 year old...please let me know cos im stressing now!!!! Rant over x