How do I deal with the emotional manipulation from my sister who is living overseas?

I have been living in the U.S. for the last twenty years. I have my own business, partner of 20 years, home, cats and my life is here. My mother lives in Scotland and my sister lives a block away from her. My sister has been my mothers care giver for the last 15+ years but greatly resents it. My mother has always used guilt tactics to make me or my sister feel guilty about not spending enough time with her. Now my sister is doing the same to me by harassing me to come over there and spend two months helping her with my mother who is “at the end of her life” as my sister puts it. My mom has a lot of heath issues and should really have social work step in and determine the best health care for her. My sister tells me “if you don’t come over for two months and spend time with your mom and she dies you will regret it and I’ll never speak to you again”. I’ve told her I will come over for two weeks but not two months as I have responsibilities here in the U.S, my life, my partner, my business, bills etc. Then my sister says “You’re choosing your partner over your mother and that’s sick”. I’m getting sick over the stress of this, constant manipulation and on top of that I’m questioning myself now! Am I a bad daughter, sister? Should I give up my life here for two months and go there. I’ve told my sister that my mom needs more than that, she needs professional help. To make it worse, I am bringing my partner with me for the two weeks. My mom doesn’t want my partner there, she just wants me. So I feel torn apart at all levels. Please advise, I’m at my wits end!