give yourself time, allow your feelings time to go through whatever process they need to. it will hurt for a while yet, even if you are unable to cry at present, don't be suprised if you suddenly do in the weeks or months to come.
grieving has many stages and everyone goes through them, but not always in the same order. feeling numb (no feelings) is part of the process. even when my partner who i loved dearly passed away i felt numb for a good few weeks, then cried, then a mixture of both, then questioning God, hating God, etc before after a few years i finally accepted what will be will be, we can not change the past, but we can certainly make the most of our future.
if you want to help your sister, but are scared of the consequences you can always call child protective services annonymously, explain the situation and that you do not want it getting out that it was you who called. they have to respect you wishes for annonyminity as well as investigate the abuse to your sister.
the numbness you are feeling is normal at this stage, but eventually you will feel something i call peace, it is a strange unfamiliar feeling to those who have been abused in their childhood, but when you do feel it you will know you have started a new chapter in your life, one where your mum can no longer hurt you.
till then be gentle with yourself x