Is it fair I have to miss spending Christmas day with my grandchildren because my two sisters won't take care of Mum for the day?

Here's my suggestion: STOP TAKING S___ FROM YOUR SISTERS AND LETTING THEM RUN YOUR LIFE!

Bring your partner back too. No reason why Cinderella should live in a love-less convent. I'm sure your Mom would be ecstatic to have him around too.

You're not a servant, and you're certainly no one's doormat. So stop behaving like one.

What country do you live in, Wringles?

Christmas does not have to have all celebrations occur on Dec. 25! Make a special plan to celebrate with your mom a few days before or after. Do a FaceTime or Skype call with her so she can watch the grandkids open gifts if this is possible. If not, let her know you will take videos of the gift opening and also have family record for her special video greetings -and tell her you will be sharing these with her ( if you see her after the 25th).
You will have more to give back to her if you don't set yourself up with such disappointments which build resentments. Also who knows what grandchildren may be thinking about why they are not being important enough for you to be with them? Kids are very egocentric because that is how they are developmentally wired and they may think it is something lacking in them that keeps you away! Yikes....
Hope these thoughts have been helpful.

Arrange for a caregiver whatever day you want to be with your grandchildren. Holidays are a higher fee as caregivers want to be with their families too. If you do not make the change now, it will not happen the way you want next year either. The caregiving charge should be paid by mom. Does she have the resources? If not tell sissies they will have to split the cost.

I went through what you are and until YOU change it nothing will happen. Next year maybe mom should be in a community, then you will not have to deal with it at all. My mom was moved earlier this year. My sissies wanted to have mom to their homes for holidays. I would not let that happen as my mom's confusion then agitation increased when in their homes with many more people than she was accustomed to. Sissies thought I was trying to control mom and them. They found out differently this year when facility did not want mom to go out because of behavior issues that are a major part of her life.

The last sentence was important. wringles, did your sisters want your mother to go to a home and you offered to help her at home? That would explain their actions toward you. Tell us a bit more about your situation.

What I don't get is why you are allowing yourself to be bullied by your sisters. You're not allowed to live with your partner? They don't help but feel free to criticize while you give up your life caring for both your parents... why do you pay attention to anything they have to say?
I think it is time to pretend you are an only child and act accordingly, make arrangements for caregivers for your mum so you can have time off. It may be too late for christmas, but make it your new year's resolution to be more assertive about your needs.

I can relate to the OP's problem, but I'm on the other side of it. Is it fair that I have to spend every Christmas with my mother because my sister always spends it with her grandkids (and my other sister spends it alone with her husband)? We all do get together for Christmas Eve, but then there's Christmas Day. It saddens me to think of my mother all alone on Christmas Day with no visitors and a microwave dinner. So I made a dinner for her last Christmas, and I already told her I'd do it again this Christmas. My sisters don't care enough to do it, that's the bottom line. Apparently I care more about her feelings than they do. It's hard to accept but that's the reality. So I'm doing it.

It sounds like there are two issues here.
1. Wanting to spend Christmas with your grandchildren
2. Feeling resentful that mom's care falls to you.

1. Arrange for your sisters to spend Christmas eve or Dec 26th to with mom. Start a new family tradition of that being your special day with your grandkids.

2. Start looking around for good facilities for mom. "They don't want" and "mom doesn't want"? Slavery was abolished a long time ago. Your mother needs care. Not necessarily your hands on care. You've done your share.

Who's in charge? If you have the authority to run the household, then set up the schedule for Christmas the way you think is fair. Bring the grandkids to your house with your mum for Christmas day. If that's not possible, then arrange for care for your mum for the day so you can visit with your grandkids. Have a Christmas celebration with mum the day before Christmas. You can open gifts, have nice meal, etc.

Is there a facility or family where she can stay for the day so you can visit with your grandkids? Explain that you are alternating years of celebrating with your mom on Christmas day and Christmas Eve. If siblings want to help, great. If not, then let them keep quiet.

Is mom aware of what is going on?

You didn't miss it yet, today is only December 17th. Focus on your own children and grandchildren. Let them do the same. Consider that your mom probably can't handle all the noise and confusion when three generations get together. Want all you have, because you can't always have what you want.