When is it enough!!

Far as I can remember my mom is always been like this but I literally cannot take it anymore. She expects the world from me and I can't give her anymore that I've got. For the past few weeks I have catered to my mother taking her to doctor's appointment, running and buying her groceries taking her to her best friend's house. You name it I have done it. Every time I tell her no she likes to call me a bad mom and a piece of shit! And will literally blow up my phone hours at a time until I answer a few days ago I was at work my mother called me wanting to know if I would go to the grocery store for her. I told her that I was at work and after I got off I wanted to go home and relax she literally called me 15 times in 20 minutes .

(She can't drive due to seizures) so finally after the hundreds of phone calls I received I answer the phone and I told her that I would be there to get her grocery list. She did give me $100 to go and buy food for her I went there got the groceries but it came up to $140 so I called her and I told her what the issue was she said go ahead and pay the difference and I will pay you back when you get here. I told her that I really needed the money back because that was for my son's birthday . She said no problem. I'll give it back to you. so I paid for the groceries and went to her house put everything away while she was sitting on the couch smoking her cigarettes. before I left I asked her for the money and she's like you're kidding me right? I said no I'm not I need that money she said I'll give it back to you as soon as I can! it's been 2 weeks now I called her up and ask her for the money my son's birthday is in just a few days and she told me that he can live without a birthday cake. I went completely off I lost all my composure I said don't you ever contact me again you always like to call me a piece of shit but really it's you . you've treated me like a dog for years and I am so tired of it. But the worst of it all is that my brother and sister is completely taking her side they said that I was way too harsh on her and then I don't know what she's dealing with and how the seizures are affecting her. of course. I felt like it was a guilt trip but she literally don't care about me or anyone else. I feel like enough is enough!! Am I wrong????