I agree with Bae Macken’s assessment—I don’t think the SIL is the narcissist in this situation. But, giving the benefit of the doubt, here’s my advice: as Dan Savage likes to say: “Use Your Words”. Identify the behavior she is doing that is annoying or distressing to you and ask her to stop. Or, bluntly, just tell her that the fact she's married to your sibling does not make you want to be her friend or include her in your conversations.
I had a narc SIL who sent one of her flying monkeys to follow me around in the way you describe here. In the past, this flying monkey had baited me into conversations and gossip by badmouthing my SIL to me, and then she'd turn around and blame me for gossiping and trash talking (my fault for having poor boundaries regarding "No Contact").
When I re-committed to "No Contact" --which includes no info gathering, no gossiping, no talking about that person at all, I had to go 'grey rock' with that flying monkey. When ignoring or giving brief answers and then physically leaving whatever space whenever she’d approach didn’t work I finally sent her an IM and laid it out for her. I just flat-out stated “I’m sorry, I’m just not comfortable speaking with you. I’m sorry if that seems rude, but that is honestly how I feel and I don’t want to speak with you or be around you,” and that was that. It is very uncomfortable and awkward to be that blunt, but sometimes you just have to clearly state your needs & feelings out loud for people to finally get it.
And then — here’s the important part—stick to it. No matter how ‘interesting’ her life gets, no matter how much you may want to participate in a conversation with her, DON’T. Stick to your stated boundary of not wanting to speak to her or spend time around her.
For the sake of family peace you may find yourself around her for short periods of time at family gatherings or what-have-you. Other than ‘hi’ and ‘bye’ and ‘nice weather’ or ‘please pass the salt,’ don’t engage or interact. A true narcissist will give up and go away eventually once they see that they cant’ get anything out of you, *including a victim story if you are mean to them, so don’t be mean. Just be absent!*
Just keep in mind that this person is family, whether you like it or not, and is going to be around for as long as the marriage lasts. So pick your battles wisely, with that in mind.
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