Hi Duska. We're facing a similar point with my husbands grandfather who lives quite close by and his daughter (my husband's mother) has to take much care of him. At the moment he's merely regressing/devolving, so not yet sick or dying at all. My mother in law feels very frustrated and angry and would prefer that he dies, because she experiences it as quite a burden to have to take care of him which also managing her own life.
The only solution I see has to do with how you approach the point inside yourself - because the physical consequence most likely can't be changed - and so in some way, the point has more to do with making peace with the situation than anything else.
Something I've seen in myself is a point of being in denial towards death and aging and the deterioration of the body, where we tend to walk around in our lives, taking life for granted most of the time and actually believe ourselves to be in control of life. When something like this then happens, we are confronted with the reality that; we're not in control of life. So this could be where the reaction is coming from, taking life for granted and wanting her to die, so that you don't have to face the volatility of life. Making peace with that - with not being in control of life - might support you to be able to embrace the situation.