You shouldnot be rude on her because she is a wodow, nobody becomes a widow by choice. But you should respect the family value, stay within limit and never indulge in any sort of sexual engagement with her. If you cant tell her upfront, start in the proper way. First of all, stop sleeping on the same bed. Secondly, if she makes any advance, ask her you are not keen. Come on, she is not getting a dick, maybe thought something remains in home is safer. You should ask her to get involve with other men of her age, matrimony sites can help too.
The third step is, fetch your girl friend in your home, spend time with her and make sure when you are hugging and kissing, your aunt can see that. You need to prove that you are a one woman dedicated man who doesnot look at others, no matter how free that is.
At this age holding such advance is very tough, but trust me, thats the right thing to do. And people do right things when they are given with proper education, brought up properly and taught high moral to maintain. Your parents are successful on that ground, dont let them down.