Keen on marrying a girl. Need advice on letter to her sister?

Sorry this will not be what you want to hear. There is no such thing as a "slight" mental illness, because if you know about it then it has affected her dealing with life already. I know you will not take any notice of me, but run far, far away from this girl. Do not think that you can rescue her, you cannot. Some mental conditions are genetic and will possibly be ped down to any children you may have. Please do not give in to this crush, it will ruin your life.