This is how prayer works: Prayer is a believer communicating to God one's needs and praises. God answers those prayers either immediate yes, immediate no, wait, or differently than requested, depending on what is best for that individual's situation.
I will pray that your brother and his girlfriend will have peace of heart about the upcoming surgery, that God will guide the surgeon's hands, that the surgery will go well without complications, and that the girlfriend will have a full and speedy recovery.
Brain surgery is serious but I personally know a couple people who had tumors removed from their brains, and they're doing very well now. One has even gone back to being a commercial pilot. There have been many advancements in the procedures in recent years, so it's less risky, and with better results now. Just the fact that the girlfriend's tumor is operable is a good sign.
