One thing I want to mention is because your mom has a brain tumor, she may not be thinking clearly. My husband recently passed away from a brain tumor and there were many times he said things that just were not true-but he thought they were. Are you getting all your infrmation about your mom's diet/sleep/care from your mom? If so, it may not all be exactly as she reports.
However, if you at all feel that you mom is being forced to do something she doesn't want to do, then, yes, take action. My experience with Hospice was that the social worker was more than willing to be the "bad guy" if needed. I think calling hospice and having someone review your mom's diet and sleep habits with your sister are a good idea. I would be surprised if Hospice says anything other than your mom should be allowed to sleep as much as she wants and she should eat what she wants too.
As my husband's only caregiver, I can also empathize with your sister. I knew the more my husband slept that meant the closer to the end of life he was-and that was scary-but me forcing him to be awake was not going to change anything. Also, brain tumors make people prone to seizures--and lack of sleep can bring on seizures. So for that reason alone, I encouraged my husband to sleep as much as his body needed. Towards the end, but when he was still "okay," he was sleeping 13-16 hours a day and even napped sometimes in between.
Good luck with everything.