I'm going to insist on an apology from my boyfriend's sister
Why? Did the sister of your boyfriend do something wrong?
I tried to ask to stay with them and she refused completely and won't allow me.
I am sorry that you were unable to get help having a place to stay. But failing to help you isn't something that a person can do wrong, but they may have their reasons and you should respect that. I am sure you have other family members or friends that can help you stay for the night?
I'm going to make them apologize as I deserve one.
Her house maybe already occupied or she doesn't trust you that well yet so I suggest that you stop needing them to apologize. If you really want to stay with them try gaining his sister's trust so that she could allow you to stay in her house someday.