Hello. I would suggest discussing the topics you have mentioned in your message with your roommates. It may be best to settle down and talk about what is happening, that is what I would do. If they get offended by such comments, then you are simply living with *****s. Another solution is to install an AC in the smaller room, I know you are on a tight budget but that may solve your difficulties.
You can also try and see why the sister is staying for such a long period of time and whether she is planning to move out anytime soon. Your roommates might need to open their eyes and realise that a room suitable for three, which is currently being occupied by four people, is not the right way to go about things (and perhaps the sister needs to leave).