Like everyone above said get a concealor which mathces with your skin tone ....... and even if she notices it just say that it was a pimple gone bad (i think there is no harm in lying when u r faced with people like that !)
And it's not your fault to have some medical condition like that anyone can get it , even you MIL !!!! All these things are from Allah ......who wants to get into any medical complication !!! It's ridiculous .........if God forbid your fiance was in your shoes how would u and ur family had reacted ??? Just dont take tension enjoy your shaadi .....May Allah give your health and solve all your problems ....:)