[ilovemyglo]

This situation doesn't seem to far away....
You've already recognized that you have a problem with this chapter of the org. and I think that is okay. You have your reasons and that is enough for me.
I also think you are quite right in saying that since it is summer it is okay, but come fall things will go differently.
It is your fraternity house, not hers. Although I am sure her sorority is welcome at your house, you can let her know (and your exec members) that this particular sister is not welcome. Some of her sisters won't come around because of this and that may bother some of your other brothers, but in the end they will get over it. Other guys that are friends with her are the ones you may want to address first. Especially if she is dating someone there.
Just approach them and be calm and let them know that although they are friends with "Betsy" (or whatever) you and her have had some problems and you have asked her not to come to the house. This way if they have anything to say (and even suggestions) then they can do it in private with you.
Come fall time and rush I know all the houses want the sororities over, but again, make sure she knows that you wish she would not attend. Her sisters will still come, at least some of them.
Now, I know that some people may feel like if their sorority sister was told not to come to a house they would not go there either, and they are sure that none of their sisters would too.
My answer to that is, I know which of my sisters could be a nuisance to fraternities. Everyone does. I always knew what girl was not thought of highly and which house. I still went to those houses and if her name came up I always let them know that she was my sister and I would appreciate it if they had something negative to say to keep it to themselves. They respected that.