I dont know if this is helpful but today I read that in some textures you import in unreal you have to uncheck the sRGB box, especially on metalic/roughness maps. Dunno if this also applies in your material for opacity or translucent but give it a go.
Replies
https://docs.unrealengine.com/latest/INT/Engine/Rendering/LightingAndShadows/LitTranslucency/