Q) How do you tell if someone is a narcissist? A) Ask them; they'll tell you. Q) How do you tell if someone is a sociopath? A) Ask them if they work for Uber; they'll tell you.
Stumbled across this from the bottom and figured you'd want to know that my cousin, who has been diagnosed with narcissistic personality disorder, when asked if she's a narcissist, says "I have needs that other people don't have and I deal with problems in my own way so it's important that people understand how they have to treat me." She conversationally volunteered that she's a diagnosed narcissist. It basically gave her cover to continue being a bitch to everyone because, you see, now that she has a diagnosis there's nothing she can do about it and it's better for her health if people respond to her as she's entitled to be rather than how they expect to.
https://www.sciencedirect.com/science/article/pii/S0191886915300167 n=2153 To be fair, the precise argument is not "narcissists will tell you they're narcissists" it's "asking narcissists if they're narcissists will give you as accurate an assessment as dancing around the issue with 40 different questions." Your sister-in-law probably isn't a narcissist. She might be a bitch, but she's probably not a narcissist.Consistent with their initial study, we find that the SINS correlates positively with the Narcissistic Personality Inventory (NPI) and has good discriminant validity from common measures of self-esteem. Additionally, we provide new evidence that the SINS may not primarily tap into grandiose narcissism. We also find that in comparison to other common personality measures of narcissism, the SINS correlated somewhat less consistently with our behavioral measure and has a higher threshold for detecting narcissistic traits. Overall, we conclude that when inclusion of established measures is not feasible, the SINS may be a viable alternative.