I am sorry you were hit between the eyes w/a very unfeeling and blunt letter....but I am not surprised at all w/her response. She's not taking sides and going to let both of you figure things out for yourself. We always say that blood is thicker than water and no matter what her son does, she will always love him. She may not like what he does, but she will love him.
At least she's not disowned the kids and wants to see them over the summer. Many don't want to even do that when the "war of the roses" is going on.
Yes, you are grieving and you will have ups and downs, but you need to pull those boot straps tight and get angry about the way things have played out. Remember...anger helps us to move ahead. Try not to stay in the rabbit hole too long and remember, each time you feel very down, allow those feelings to wash over you and release them. They will help you heal as you walk the path.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.