@tigerlilly2 Now that you have clarified things a bit more, I can definitely say that Grandma has a huge problem. Her jealousy of your mother is tainting everything in her life. That your father gladly goes along with this scenario, and just leaves your mother sitting there at home, is kind of disgraceful.
Not sure what you can do to change Gram and Dad, but you can show your mom the best Christmas day ever, before, during and after the time that your dad runs out and leaves you all hanging. I’m so sorry that this is happening.
If I were you, I would consider writing Gram a letter telling her how hurtful it is (and has been all these years) to you and your sister and your mother that she treats your mom like an outcast. Tell her that you would love it if she would come to your house and make merry with your whole family and if she chooses not to, then it will be her loss, but let her know how painful it is for you. Then ask her politely what she thinks can be done to solve this problem? If she says nothing, or pitches a fit or tells you how much she dislikes your mother, then leave her alone and just make the best of your own Xmas celebration with your mom and sister. Let dad continue to do his own (albeit disrespectful) thing. Again, sorry.
This whole thing really hurts to read, because I have some family members who are feuding with each other (and have been for about 10 years). We see half of the fueders on a regular basis (they’ve called for a truce), but the other half left the state (and would prefer to continue to be angry at the other group) and we haven’t seen them for 4 years. There are kids involved and it permeates out to all of the relatives (who aren’t even part of the fued) because we know that we can never be ALL together. It stinks.