Take a look at this thread: https://forums.unrealengine/showthread.php?4732-mirror-material-creation 
"There is a level called Reflections in Content Examples project. Open it and then head to 1.7.
What you see there is a SceneCapture2D actor facing the direction you want reflected. Once you set it up, create a TextureRenderTarget2D in the content browser and insert it to SceneCapture2Dās Texture Target. And then use that texture in the material of your mirror mesh. You can also see the material setup in that same project. "