I'll say a prayer for you sister.
Remember this...although any brain surgery can be very dangerous...the technology in the medical field has come a very long way.
Cancer surgery can be scary for the family who has to sit and wait. I've been there myself during my mother's struggle with breast cancer and an anyeurism. She went into surgery 3 times. Two of them were supposed to be dangerous and one was supposed to be routine. Ironically, both of the dangerous operations went quickly and without a hitch. The routine surgery went alright as well, but took longer when it wasn't supposed to. I think when the surgery is considered to be more dangerous, the doctors tend to take it more seriously and therefore end up at the top of their game, so to speak.
Things will most likely go really well for your sister tomorrow. My mother was very weak and sick when she had her operations, but she had the will to recover. Give your sister all the encouragement you can and talk to her about all the great things she has to look forward to once all this is behind her. Talk about specific things you would like to do with her or places you want to go together. Making sure she's in a positive frame of mind when she goes into this will go a long way.
Good luck and please let us all know how things went.