Honey, they will cry... and so will you. Everyone will. It's normal and it's expected. All you can do is hug them and be there for them. You don't have to say a word or do anything special. It's important to just be there.
God bless and keep you and the entire family.
A phone call will do. Just you being there in any way will help. Cry with them over the phone and send verbal hugs to them. It will help.
I am so sorry that you are having to deal with this situation. You didn't say how old you are but it doesn't matter. Death is hard to accept, no matter what your age. As the others have said, don't be afraid to cry. That is part of the process. But also don't be afraid to share any fun and good memories you have of your brothers and sisters mother. It doesn't hurt to laugh or talk about things that happened in the past that made an impression on you. You also might write some of these thoughts down and send them to your family. It will help them and you also.