<human>: Could you distill the essence of the text into 1-3 sentences?

I'm a junior in college that lives in a six bedroom house with six other girls who are also juniors at my college. I've known 3/5 since freshman year, two of those three are on the softball team along with the other two girls in the house.

The house is separated in to two floors, three bedrooms on each floor. I'm on the first floor along with The Girl who let's her Sister stay with her. Now, I had no idea this was going to be a thing. When we signed the lease and leading up to moving in The Girl never mentioned that her Sister would be staying with us. At all. I only found out once I officially moved in during August when the Sister had already started staying here.

Now, the reason this is a thing is because Sister is going to grad school at our college and I guess she has no where else to stay Monday thru Thursday when she has class. Personally, I think that The Girl is under no obligation to house her Sister because it's not her fault Sister didn't make adequate living arrangements for grad school, and Sister shouldn't expect to live in our house for free just because The Girl lives here.

Sister doesn't pay rent, doesn't contribute to household chores, and is not on the lease. I'd maybe understand Sister staying here for one week a month, but every week until the end of the year? Hell no. Also, it's not like Sister makes living in the house more difficult, but having an extra person underfoot during the week when I have to work and go to class is not fun and not what I signed up for.

I've already called for a house meeting tonight to discuss this issue, but I'm wondering how I should approach The Girl about it? As stated before, Sister hasn't necessarily done anything wrong but I just don't want her in the house.
<bot>: Housemate has her sister staying in her room for 4/7 days of the week, was not told about this AT ALL before moving in. How do I approach housemate about it without seeming like a bitch?