I'm 27 and recently lost my mother. The best advice I can give you in the short time I have at the moment is do whatever you can to get yourself there as soon as possible. The time before the surgery is just as important as the time after. I have teenage sisters as well and they will need you just as much as your mother will.
So in short, get in the car. You don't want to lose any time with your family before her battle begins.
I was away in Maine at college when my mom was first diagnosed so i know how hard it is to leave your life. I had to have an uncle pay for a ticket for me. Just explain to your boss, landlord and whomever else what is going on and provide documentation if they want it and you should be fine. Just remember the most important thing is being with her and your brother and sister.
you will figure it out. Just get there and things will be easier. I am in no way suggesting that your mother will not make it through this. As you can see from the people on this board many many people do! If your mom is anything like mom she will fight like hell. Its great your grandmother is there for her. If your grandmother is able to focus on her care you should be there to back her up and ask questions. You should focus on being there for your sister and brother. I am the second oldest of 7 so I have seen kids handle this at all ages. Be there for them it's the best thing you can do. We can't actually fight the cancer for our mothers but we can help to make it easier for them.
you found the right place.
more later
Amy