I would suggest installing some tile directly above the sink base. Even just one row of a subway tile or glass tile. The water will splash the wallpaper and ruin it over time.

HI -- You could have the mirror rest on the back of the sink or have a cut mirror made to rest on the back of the sink if you haven't put a mirror up yet . But just how much are people going to be splashing around in there ? If the powder room is mostly for guests and for just washing their hands , that should not make such a water hazard for the paper .