Well, that doesn't sound like sisterly love to me. She's your older sister? She should be setting a good example by showing kindness since your parents have shown kindness to her. I do believe she is the baby sister in attitude!
If you can afford it, a hotel sounds like the easiest solution. If not, you will have to decide how much peace you are willing to display with your family. It may hurt you in the short term, but if you can find a way to overlook your sister's rudeness and your parents inability to stand up for your "visitor's rights" to good manners, then stay at your parent's in the too small room. But for your own peace of mind and future relationship with them, you need to discuss the guest sleeping situation with your parents and sister. Your parents are the owners of the house. This should be their decision, not your sister's. If they don't offer you the other room, that's their decision and you have to decide if you can live with it or not. Good luck! I hope it works out well for all of you.