I have money to help my mom... I just don't want to

My sister told my mom that I got my tax refund. I got 8,000 back and my mom called me asking for help because she's behind on rent. I don't want to help her because I know it will take her a while to pay me back. I've helped her in the past and it feels like she never appreciates it because of i help her all the time the one time I don't she gets mad and says I never help her. That I'm a bitch. Last time I didn't help her she told me fuck you and she hopes "God" takes me kids. Then her excuse is she was mad and she only said it because they aren't my real kids. They're my step kids. Even though I've been their life for 9 years. I'm just overwhelmed