I can sure see how this would wear you down after years of feeling copied. But I also find your sister-in-law frankly sad and pitiful, more to be pitied than disliked.
She must be either so enamored of you and what she sees as your great ideas that she does not even try to come up with ideas and activities of her own; or she is so tragically blank when it comes to having her own thoughts and feelings that she just copies someone else. I wonder if her family history would show that even as a kid and teenager, she copied other kids? I have a feeling it would. She sounds insecure and she may feel that she's just a cipher, a kind of paper doll person who has nothing of her own going on, so she sticks like a barnacle to whatever is suggested to her -- and yes, I know you're not "making suggestions" but are just conveying information; however, somewhere in the depths of her mind she thinks, "What a good idea. I'll do that too."
Can you possibly, after all these years, try to see her as pathetic and sad, as someone who may have a fundamental personality problem, as someone who may be deeply insecure and not really know herself or own mind? If you can, that may help you to brush off her copying. She may indeed see it as her flattering you, even if you don't. Show your own confidence in what you do and say by ignoring the copying and saying, "Oh, poor Sally, if she's happy doing what I do, good for her because she doesn't seem to be able to come up with things on her own and that's just sad." Say this to yourself - not her, because she won't see it.
It does sound like there are other, deeper issues here between you and your brother, too -- you say he, not just she, must be the center of attention, and seem to resent the idea that both are "at every event" if a relative visits (you say you are OK with those things -- but why bring them up if you're really OK with them at the deepest level?). Your issues with her may be tangled up iin a lifetime of issues with your brother.
I guess the key here is this: Do you like them as people? Would you be interested to talk with them if they were not relatives? Do you share any interests or opinions or history with them that makes you enjoy their company? If not -- limit your contact with them and don't talk about your life and your activities when you do talk; talk about the weather, TV, their kids, whatever. If you DO actually like them on some level, then find activities to do with her--specific things to see and places to be, so that you are doing less chatting and more activity. But it sounds to me like this is a case of "they're family and not folks I would really have as friends otherwise." In that case, limit contact, arrange separate events with visiting family members ("Oh, our schedules just clash this time so we're seeing X on our own") or whatever.
But mostly she sounds pitiful.
