Hello ciao,
I am sorry that your mom is having these reactions. It is completely to be expected: she is facing the loss of her husband, her life-partner...the man she spent her life with...the man she leaned on for support in all aspects of life, as he did her. She knows she is facing an uncertain future, and despite knowing you and your siblings will be there for her and each other, she will use that close, familial trust and support to express her most heartfelt emotions - good and bad. I would encourage you to remain strong...resolute...and consciously make effort not to take what your mom says "personally", as she may lash out at times. She needs sympathy, empathy, and unqualified acceptance and support for whatever she is feeling and expressing.
I don't know what faith you may have - what faith your mother may have - but I would encourage her to seek counseling of some sort, both alone, but also with you and your siblings, and most importantly with your dad present. It will be a very "grounding", "centric" experience for your dad to be able to express what "HE" feels, and what "HE" wants for your mother and for you and your siblings when he is gone. It will be a chance for him to set expectations in your mom, you and your siblings. It gives your dad a sense of control in a situation where he otherwise will have no control. It also gives your mom and you a new goal to try to meet his desires for what happens after he is gone. And...as always happens, each of you will go through the five steps of grief as outlined by author Elizabeth Kubler-Ross in her famous book "On Death and Dying": Denial, Anger, Bargaining, Depression, and Acceptance ("resolution"). I wish you well in your support of your mom AND your dad. If he is able to physically, I would encourage your mom and dad to take a wonderful vacation together for a week or two...something to offer them both a wonderful memory, and chances each day and night for important, meaningful conversation and closure. Good luck to you.