Need some advice my brother was killed in Iraq and now his wife is coming onto me....?

My brother and i were twins so as you can imagine extremely close we could finish each other thoughts it was almost creepy to a point. Well my brother married his high school sweetheart (My brother and i are 24). They have two son's a 2 and 4 year old. And i love my nephews more then anything there both so adorable. Well i am engaged myself and i love my fiancee so much and she loves me well my brother was killed in iraq around 3months ago. Its been super hard and my fiance has been amazing through it all and been helping me so much. Well my brothers wife when i was over visiting her and my nephews the past few days have started to come onto me and she did it today right in front of my fiancee and i tried to tell her how inappropriate it was and she started sobbing. Now my fiancee doesn't want me going over anymore. I do not understand why my brothers wife is being like this. I love my brothers wife like a sister but she doesn't seem to understand any boundaries. And i want to be able to see my nephews without her coming onto me. I do not know what to do. I do not want to hurt her or anything but i am not going to jeopardize my fiancee and i relationship. Advice?