Alzheimer's or even progressive dementia can cause dramatic, unexplainable, and unpredictable changes in the victim's outlook and personality. If this animus towards you and your sister began after your mom began feeling the effects of her disease, maybe you can chalk it up to the disease itself and choose to believe that it doesn't reflect how your mom would have felt if she had been in her right mind.
Just because she recognized you and had certain memories of you doesn't mean her attitude towards you wasn't entirely due to the ravages of her disease. I have an elderly maternal uncle to whom this is happening right now. He has dementia, not Alzheimer's, but at this stage it's more of a distinction that a difference. His sister- and brother-in-law have bent over backward for years to help him and my aunt out in numerous ways. They have gone way beyond the call of duty or even familial love. But in the last year or so, my uncle has acquired an almost venomous hatred of his brother-in-law, for absolutely no reason whatsoever. When they come over to visit or, more often, help out, my uncle sulks in silence, clenching his jaw and fists, and glaring holes through the man. Again, there is no justification whatsoever. My uncle accuses him of all sorts of terrible things and even denies that he ever had the long and successful career that he most certainly did. No amount of explanation and proof that they were the best of friends for years does any good. But we all know that my uncle would never feel or act this way if he were in his right mind. Alzheimer's and dementia can be expressed in a myriad of subtle and contradictory ways, besides the obvious and expected symptoms. They are a true scourge of humanity.
This could well have been what was going on with your mom. Maybe it will help you and your sister feel better if you can find it your hearts to give her the benefit of the doubt, and choose to remember her as the mom she was before she was attacked by this horrible disease.
I am so sorry for what happened to you and your family, and wish you only the best. Hang in there. You are not alone.