Sibling negativity

I am a 51 year old female, Multiple Myeloma survivor. I am doing very well with regard to managing the cancer and am in complete remission. I am, by nature, a positive person and this has helped to carry my through the past 7 months. My immediate family and friends have been very supportive and loving.
The problem are with my two sisters, who choose to constatnly spew negative thoughts in my direction. I can do nothing right in their eyes. They are relentless and now one of my brother in laws has gotten involved. I can not even temporarily walk away from them, as much as I wish I could, as I am my father's advocate. My Dad is in a nursing home and I was handed, by my sister, the task of being his advocate shortly after my diagnosis. I also am the only sibling still raising children, who works and is fighting cancer. It doesnt seem to matter to them. I email them with everything that is happening with my Dad. They question my every movement regarding my Dad. They have no regard for my situation. I love my father with my entire heart and will do anything for him to insure his care is the best it can be. My husband is very supportive of me and I dont know what I would do without him. The stress brought on by my sisters and caring for my Dad is incredible and I dont know what to do. I am at a loss. I have to figure out a way to handle this and I am at a loss. Any suggestions?