Do I have the right to be concerned?

Yes, you do have the right to be concerned. An eight year old is learning how to read and certainly is curious about her surroundings. Your daughter should want to protect her little sister and take proper care of her. She should speak to the landlord or the owner of the posters and ask to have them removed. I don't buy it that she has no say in this matter. And why would she want to see those posters herself? replace them with appropriate pictures. If she refuses tell your daughter to visit her sister in your home.