How do we tell my mom she can no longer live alone?

My mom has lung cancer.She was in remission for almost 2yrs.when we found out it had was in her brain.She had WBRT and she now has worsening Dementia.She is basically an invalid and struggles to get to the bathroom unassisted.This last hospital trip the Dr.informed us (we were already making plans)that she couldn't stay home alone.We had someone at her house,including myself,a minimum of 3hrs. a day.This is no longer enough and I can't do it anymore as she lives an hour away.She has said (when the time comes) she would be happiest with me.She is in rehab right now.I have no intentions of letting her go home alone again.I am glad the DR. spoke up and is on my side.My sister is here from out of town now so we have to tell her now.If we had someone at her house 8 hours a day it wouldn't be enough.I can't drive the 10 hours a week for all the Dr. appts. the financial stuff etc.I do work more than 40 hours a week.I cant do the 15-17hr days anymore.When mom got sick there was also a 90+ grandma I was looking after in assisted living.Laundry and grocery.Little by little my life has gone from me until it was no longer mine.I want to bring her here for as long as possible.I am aware this is probably not a permanent solution.I went thru the checklist on this site,for things to look for indicating your parent needs help at home and she had all of these but 2. The biggest problem is her cats, but have made arrangements for someone to take them.I know how hard that will be for her but I have 2 myself and there would be big fights.My cat is diabetic and nobody would want him.She can't and hasn't been able to care for them or herself for some time.She sleeps 80% of the time.She won't even put food in the microwave for herself.She got her 3rd uti and dehydrated.I have called many times a day to ask her to drink water,take her pills ect.I set up an am/pm pill container and even with constant reminders she hasn't taken her pills either.My plan is to bring her here and have somebody come while I work.My husband 100% on board,his idea. I have read all the blogs and know what I'm up against.It will be easier then being so far away.I and sis are just scared to tell her.I might add that she is a picky eater and there is nobody but me that can cook for her. Please if anyone knows of an easy way to help us tell her or convince her I am only doing what is best for her.