[33girl]

First off points for it being a two bedroom. If you do end up doing this, as silly as it sounds, I would make it so that each of you has your own room.

As far as it being "improper" in the moral sense, your sisters need to crank the redwood trees out of their asses. It's no more improper than them screwing a guy in a car and then going home.

As others have said this isn't the dorm. Shacking in the dorm isn't "living together." Unless you have a very enlightened or very Felix Ungeresque boyfriend, more than likely the housework and cooking is going to fall on you. Especially if he has a busy grad student schedule. Do you really want to take the responsibility for that?