My sister Elizabeth is having boy troubles. Any advise I can give to her?

She is 18, so considered a grownup. Her boyfriend proposed and she thinks she likes someone else. With the other guy, she's talked and stuff with this other guy, and she thinks she has a connection with him. And sometimes she says, "he seems to have a better personality then her boyfriend" and "she knows her boyfriend isn't the one". She told him that she will think about it. But she don't want to tell him she has been seeing another guy, because he will be mad out of his pants, but then she don't want to get married, and stuff, then tell him she isn't in love with him. She wants to know what to do. Any advise I can give to her?