Sorry to hear your grandma is very sick. Based on my experiences with sick family members, it's important to talk to your mom and set boundaries! Ultimately, everyone wants your grandma to be taken care of and feel better. However, you need to share the time sacrifice required to support her, or as @NickHoward suggested, find someone to take care of her.
My biggest suggestion however is to talk! Communication is the key to making sure everything is out in the open. If it helps, you might want to consider involving a third party to facilitate your conversation. I'm sorry you have to go through this, but hope this can help a little.