Open your material in UE and click on the material output(Biggest rectangle with many circle in his left side) after then search “two sided” on the left side on the screen and check it on, now save your material. Or just add thickness modifier to your mesh in Blender
Edit:
Or your mesh is has thickness but the normal of the faces are incorrect. If this is the case. In Blender select your mesh, go to edit mode, select all the faces, and Shift +N. After then you can check it in Blender. Go to Viewport overlays(Top right side), and check “Face orientation” Blue is ok red is wrong
Thanks Bro, I try and both options works!!! also the normals are fine! Gracias Totales!