Nope: http://universesandbox.com/faq/

  Does it account for relativity?
  No, the physics in Universe Sandbox ² is currently only
  Newtonian.

  Why?
  The short answer is that you need a supercomputer to
  accurately simulate general relativity.
  Jenn, astrophysicist and Universe Sandbox ² developer,
  explains more in a blog post: "General relativity 
  requires simulating the spacetime itself. That is,
  taking your simulation space, discretizing it to a 
  hi-res 3-D grid and checking the effect that each and
  every point in that grid has on all neighboring points
  at every timestep. Instead of simulating N number of
  bodies, you are simulating a huge number of points.
  You start with some initial data of the shape of your
  spacetime and then see how it evolves according to the
  Einstein equations, which are 10 highly non-linear
  partial differential equations."

  We are, however, interested in adding in a few features
  which would address some effects of relativity. One 
  example is setting gravity to travel at the speed of
  light, instead of instantaneously taking effect as it
  currently does. You can read more about these in 
  Jenn's blog post: Gravitational Waves & Universe
  Sandbox ².