How can I look fabulous at wedding without outshining the bride?

Boyfriend (oddly) scared that I'll buy something that'll make me look look better than bride (his sister), cause I wanna look fabulous since I'm spending $ (don't wanna outshine her). Unsure about wearing black cocktail cause bridesmaids wearing them. The bride will either love it if I wear b + w to co-ordinate w/ the color scheme, or hate it for taking away attention especially since sitting w/ family, idk which one. Early or mid afternoon - late evening wedding. Not sure what length to wear (thinking tea-length or slightly below tea length?) Not sure if open back appropriate (will not wear shawl whole time). Fall wedding. Extremely pale w/ dark-dark brown hair.