What devastating news you have had - this must be a terrible shock and I really do feel for you. It is very common in situations like yours that people around you feel very helpless and don't know what to say or do. That's a very difficult situation for you, because more than ever you need people to talk to and who can give you support. Exactly who to talk to or who to tell is very much up to you, but it should be people you trust and that you know will be there for you. It may be that you can talk to one person about some things and another person about something completely different.
Although you probably feel completely down and need people to come to you, it may be that you have to take the first step in talking to people. It seems like you have not told your friends about it yet, but even if they did know, many would be very uncertain about what to say and some may even try to avoid you because they are so scared of saying something wrong and upsetting you more.
We would normally recommend that everyone involved in your life in an important way should know what you are going through now. This is the only way they can support you and you do need people to talk to and lean on. Although no-one can take away the pain and sadness of what is happening to your mum and all of you in the family, it is usually a big help to have people around you who knows about it and that you can talk to whenever you need to. It is important that you tell them what is best for you - you may not want to talk about what's happening to your mum all the time, sometimes you need to be with your friends just to get away from it all for a little while so that it doesn't overwhelm you completely.
When it comes to the questions you have, but are scared to ask, what you are saying here is very important - that you realise that although lots of questions are there, you may not be prepared for the answers. All this has happened very fast and you need to make sure that you want the answer before you ask. Usually though, we find that although many answers can be not what we wanted and very difficult to deal with, it is still better to know the situation the way it is. Some questions may be medical about your mum's condition and what is likely to happen and hopefully someone in the family will be able to help you with those answers.
In general, it is better to talk openly in the family about what's happening. Often people try to protect each other by not talking about what's going on. However, everyone is still thinking about it all the time and instead of sharing the burden and sadness, everyone is left on their own to deal with it. I hope you will find the support you need from your family, friends and other people in your life. Thinking about you.