I have to agree with benji. You may regret not spending time with your Mum when she is gone. I can understand how you feel though, as my own Mum became very difficult and unappreciative of all I did for her, when she was ill, with mental issues, (not Parkinson's). I sometimes cried at the hurtful things she said, but I felt I owed it to my Dad to support him, during this terrible time. I too, had other things going on in my life.....dealing with an alcoholic husband and the break up of my son's long standing relationship. This was all prior to me being diagnosed with Parkinson's, but I was generally feeling unwell, at the time.
Does your brother sill see your Mum? Perhaps you could agree to visit her just once a week. Do you have a PD Support Worker who could talk things over with you?
I think the title of your post 'A daughter's love', shows how torn you are and that you still care.
No-one can really advise you how to deal with this situation.....only you can decide what's best.