Help! i cant stand my bf's brothers gf

Ok, I cant believe that I am asking this question, but here goes.
I have been with my bf for 2years now, we are blissfully happy. We had our own place, but I got laid off and his temp job ended. We ended up moving in with his parents and younger brother by 6 years. The first month his brothers gf basically lived in the same house, even though she has her own place or lives with family. This caused me great anxiety, to which I expressed to my bf on several occasions. I would talk to his parents about "budgeting" out the meals, and she would just come a basically squat and eat, shower for days at a time. This isn't the best part, she has a kid, which non of the family has meet except for the brother. So she was here and not with the kid, that was under the age of 2.
I have agreed to help clean and cook in the house, because they are allowing me to live here, which I feel I should contribute as much as I can. She comes over and showers, and has nasty bathroom habits when it comes to her feminine hygiene products, which is disgusting to me. She leaves them in the trash, which is what she should, but does not conceal them in any way. I have expressed this to my bf, I keep trying to tell him that I do not feel that if she sleeps here consecutively for more than 3 days, she is not a guest. I do not feel I should have to clean the bathroom after her. Plus the dog likes to eat them and throws them up, which I got to clean up one day when everyone was gone at work.

Now, last weekend she finally brought her kid over, who just turned 2, I guess they have been dating for a year now or close to it. I do not understand this, why she hasn't brought the baby over sooner. I feel, and this is the real reason why I issues with her, she treats being a mother like a novelty. I know that the baby's daddy gets her on weekends, 2x a month. But this chick comes over every weekend, with no baby in tow. Randomly she is here during the week, never the same days either. I have real issues about her playing house here with the brother, and going home to play mommy. My bf knows how I feel, I tell him that I dont dislike her, just do not like how "they" are acting. She is a mother first and a gf second. Besides the fact that she had a baby under the age of 1 when she met the brother, I know things happen in relationships that we cannot control but come-on. The first thing on her mind shouldn't have been jumping in bed with another man when she has an infant at home.
I have told my bf that I really dont know her well enough to not like her. When she is here, until recently, she stays in the brothers room because the dog in the house didn't like her and would bark non stop when she came over. Another thing is they have to lock up the dog when she brings the baby over, the dog is old and doesnt see well plus shes never been around any children. Which my bf doesnt like that at all, that his brother locks up the dog and allows the baby to run all over the house. Which makes the dog all the more irritated and barking, she is too old to understand why she is being locked up.
The parents are very sweet to me, and for the longest my bf's Mother didn't even speak to her either. She would roll her eyes when she came over. She asked me one time about the dog and if that ever happened when I started dating my bf (the dog is his) I told her no, because I would never allow a dog to be aggressive towards me.
I seriously have been asking everyone I know what I should do about this situation. I am not rude to her, I just really dont have anything to say to her. I have talked to my bf about this, I told him that I do not want this to cause us any problems, bc I am trying to get along with his brother and gf. But they are secluded in his room all the time, and on the rare times they are in the living room or where ever other then his bedroom, it is awkward at best. I know that my bf is tired of her being here all the time too, but he is not the person to complain.
I can only hope that we are able to move out soon. I fear for my relationship with him because of this situation. I know he loves me, but at the end of the day that is HIS brother. I guess I would have said something to my sibling by now if the roles were reversed, that is how my family is. We dont hold back. My family would respect the truth over lies. I started a journal to complain to, but quickly ended that idea because all I was doing was typing furiously on my laptop when she was here... It seemed more like a daily minute by minute of the crap that irritated me.
I know that the following statement will cause several people to have an issue with, but this is how I feel. I was raised in a blended family, to this day my half sister treated me differently when it came to her Dad. I can honestly say that the majority of people I know that have blended families or have came from one, they all say the same thing that they would never change the siblings but for the parents made it impossible. Either it was the new Mom or the new Dad. So again, I feel that the baby is going to be the one to be hurt by this entire situation. I know that in the end what the brother does will never have any affect on me, and he is an adult.
Thanks for listening