sither-in-law
The sister of one's spouse who acts in a purely selfish and ultimately evil fashion, working to create strife within a family to their own ends, whether emotional manipulation, financial gains, or perhaps both. She has the same motives and machinations as the Sith, or Evil Jedi, from the Star Wars galaxy.
"I found out my sither-in-law has been actively seeking to turn family members against my husband and I, hoping that it will somehow result in her getting a larger inheritance."