Let's put things into perspective... Does she holds the values of your sisterhoood? Do you think she will appreciate that you will not oppose to having her in your house? Is there any possibility you will bump into her? Do you really think "he" is worth it? Do your really think their values in their relationship are stronger than your values and your house's values?

Move On! Enjoy your ADPi life in college. Perhaps, she is not even thinking about you know. Does she likes to stalk? Don't hesitate to put a restraining order, they are for "psychos".