I am so very sorry for your loss. One thing that may be happening has to do with the fact that in addition to the psychological struggle to get through this time your body is likely to be going through a hormonal roller coaster.
I would strongly recommend psychotherapy for coping with the loss you feel from the miscarriage, helping adjust yourself to your sister’s pregnancy and her lack of effort at support.
This is the time for you to recover and reclaim yourself through support through psychotherapy. See if the hospitals in your area are offering individual or group therapy for grieving. This is a profound loss and there are people available who can help.