I think it probably doesn't matter until they are old enough to decide for themselves. I suppose you get some 'cute' factor when they are really little from dressing them alike, but after a certain point, their individual personalities will start to blossom. Having said this, there are families where same sex singletons are dressed alike, except for the colour. (Eg Sally likes blue so always has the blue dress, but Jane likes pink so always has the pink one). Personally I suspect grandma's are the culprits here.... Each to their own, I guess!