No, JJ Abrams & Andy Serkis confirmed that neither are Sith.
"Kylo Ren is not a Sith. He works under Supreme Leader Snoke, who is a
powerful figure on the Dark Side of the Force." - JJ Abrams
Speaking with EMPIRE, The Last Jedi star said: “He’s definitely not a
Sith, but he’s certainly at the darker end of the Force. Without
giving too much away, that begins to unfold a little in this one." -
Now there are not a lot of instances of dark-side force-sensative users whom are not Sith, although Asajj Ventress might fit the bill being a Jedi padawan turned assasin who just comes to work for the Sith, but there are light-sided ones such as Maz Kanata (New Trilogy), and an ancient Jedi splinter group: Ordu Aspectu (Doctor Aphra), but there is also The Bendu, a representation of the center of the Force (Rebels).
It makes sense that there can be the same thing for the Dark Side,--granted I'm not sure if The Last Jedi makes either Snoke's or Kylo Ren's "end game(s)" and overall philosophy perfectly clear, as much as it makes one question what everything is.