MIL issues over my husband's health

My husband has recently been diagnosed with Stage 3 testicular cancer. This is a major blow to our family on so many levels and we are pushing through the best we can but I am having some issues with my MIL. My husband is terrible at dealing with all of this medical stuff.. he has completely shut down and I have had to take the reigns when speaking with doctors, making appointments, sorting out things with insurance, etc.  I have done and continue to do everything in my power to take as much control of the situation as I can. My husband has had total faith in me but his mother stepped in and started to complicate things.. I firmly believe she is trying to convince him that I do not have his best interest at heart and that he needs to let HER handle things. She has even made comments to him in front of me about him making her his power of attorney and making her legally able to make decisions over me incase things take a turn for the worse. Now I totally understand this is her son and she only wants what is best for him but I have never given this woman any reason to feel I wouldn't care for my husband appropriately. I feel this is just a "control" thing that she needs. She feels powerless and can not stand seeing another woman care for her child. I try to put myself in her shoes and think what if this was my child... so I have remained respectful. We have invited her to appointments and have included her in making decisions but she has become so over bearing. Any advice on how to handle her in this situation?