I don't think what was said was necessarily coming from a place of cruelty. It was certainly harsh, but it could have been her way of being kind. Acting like that would ensure Lippe would be stepped on by other nobles, and if the medicine would only prolong her mother's life, not cure her, then running dry all of the family finances, connections, good will and reputation in order to keep her alive that much longer would in no way be what her mother would want. Her mom WILL die at some point, and from what we've seen from her, what she would wish for the most is that her (surviving) children could live on well after she is gone.
Denying the funding and calling Lippe out for damaging her own future in order to stay in denial about her mother's impending passing, instead of spending every moment that she can with her and ensuring that her mother is confident she'll be okay once she's gone("focusing on what she CAN do), is probably the kindest thing that could have been done for both of them. The issue is that Lippe had been hit by so many horrific losses so rapidly and so much grief that a harsh message like that was beyond what she could process.