I am so sorry you are having to go through the challenges set before you on your own. It is a devastating feeling when siblings abandon you, and also when you have difficulties with your mom as well. I would suggest that you get some counseling or advice in ways that you can approach your mother and communicate appropriately to her your needs, the expenses you have taken on, etc. By coming from strength, you may be able to show her the importance of helping you out as you are caring for her.
It is a difficult road to travel, but have faith that you will be guided to answers and situations that will benefit you as well.
Blessings to you. Gail