My sister came onto me?

We was at a party, and my sister was drinking quite a bit, she remembers some things that happened but not all of it. We are not biologically related, she is my foster sister. She was going to bed and I was looking after her all night because she had been drinking so I made sure she was alright and she gave me a cuddle good night. Then she kissed me and she wouldn't stop. She started putting her hands down my trousers and I was saying that that wasn't going to happen. What am I supposed to do about it now? Coz it felt good in a way but because she is my foster sister, it is rather awkward. Please help.