How do you deal with a sister who said she would help with our mother (who lives with me) and now will not?

we talked about this before my mom came.she said she had no problem helping.my sister does not work.i work full time so does my husband.Everytime i have tried to talk to her she flips out on me.she blocked me from calling her because she can't DEAL WITH IT. my husband and i are doing it all. its putting so much stress on us.my mother is not ill now but i'm worried that she maybe starting with alzheimers,she is getting forgetful and has gotten nasty with me.i have talked to her about it.she says she is sorry. my father has dementia and is in a facility.we visit every weekend.my sister was supposed to take my mom once a month so my husband and i can relax together,but i've had to push her into keeping her word.she flipped out when i asked her to take my mom in between xmas and new years because i had off of work.she said she needed a break.i told her fine(we were texting at the time)to forget i even asked i won't ask again.she said some very bad things,kept txting me all day at work.i did not respond nasty at all but i told her she needs mental help.shes since blocked my husband and i from calling her.she hardly sees my mom at all.shes supposed to take my mom to the dr. appointments and is not really doing that,my husband had to do it.i'm so stressed out i can't take it anymore.she does call my mom everyday and has taking her 2xs on the weekend since new year.but my husband works 2 saturdays a month until 8pm and i asked her to take mom on the weekend he has free so we can go out,but she won't so were stuck.i'm at my wits end.by the way i'm 61 so is my husband.my mother says my sister is not as strong as me she always makes up excuses's for my sister her whole life.my mother has been here for 2 years. my daughters(i have 3,they are triplets age 38) have been wonderful and help when they can,they have their own children and all work.my sisters children do not even call my mom and my mom is not allowed over their homes.they do not want to deal with it either.right now i don't care if i ever see my sister again. any suggestions