omg, you have the patience of a saint. if i were in your position, i'd have to tape my mouth shut to stop myself from telling her off or to get her own !@#$ing taste. going to greece with you!? same furniture and haircuts?! oh my good god. she sounds like a girl i went to high school with... she had no personality of her own, but merely absorbed everything.. EVERYTHING about her bff of the moment. it was flattering at first, but got annoying really quickly. she was a spongey leech (i wonder who she's copying now)...
i don't think the solution necessarily lies in you learning how to deal with her... she really has to stop doing this for her own good. and i don't think she's going to stop emulating you unless she finds someone else to copy or until you say something to her. and it sounds like she's been doing this for a long time, so she's bent on copying you until you let her know it's not cool.
is your brother-in-law aware of this? he might write it off as being silly, but if i were him, i'd be weirded out.
how close are you with the SIL? is it possible to take an extended break from her? if not, for the sake of your sanity, i think you might have to sit her down and let her know that this makes you very uncomfortable
Sounds like she needs some professional help. Maybe she needs someone to help her figure out who she is rather than who she wants to be. Definitely at least confront her about this or else this just might keep going on and on and on.
if i were you i'd keep my distance from her. I have a SIL thats kind of like that towards me and my boy. our two boys are the same age and everytime she see's something on him or buy him she wants to know from where and wants to get it for him. i just tell her " i forgot" or " i dont know, it was a gift". i also now keep my distance from her. that really sucks cause i love my brother to death and he's so cool to kick it with, but because she can get annoying, i keep my distance.