What was her reason for not contacting you? Was it your mothers wish that she do it that way? If so than you can't blame her. You need to find out why before you jump to anger. Also, were you speaking to your mom or did you have a falling out? Because you can't get mad at her over you and your mothers relationship problems. I don't know what your situation is but, try to take step back and look at things from her point of view. I would hate to see you lose a relationship with your sister if it wasn't her fault.
I would be totally crushed. Losing your mom is the hardest thing in the world. I don't care what your relationship was, she was still your mom. I am sorry that this happened to you. Sometimes, our families betray us in ways we never imagined. I'll tell you right off. You will grieve your mother's death for quite a while. Don't be surprised if it throws you for a loop. I know you wish you could have been at her memorial. The good news is you can talk to your mom right where you are. You have my deepest sympathies.