Is my sister selfish towards our mom or family?

My mom is an elder person and since dad passed away 11yrs ago im the one in charge of mom with my brother so we all live in the same house. My sister is married so she lives on her own obviously. For the end of the year holidays my sister always spend it overseas, never in the country, even when dad was alive it was the same. This year was the exception as she did not travel but she will travel in Feb to the USA instead. So for the 24th this year she did spend with the family (cousins, and aunts) Christmas. For my sister Christmas is more important to spend it with family and for New Year it is not very important to spend it with the family because for New Year, families gets divided and spend New Year on their own, some with families but others on their own (friends or other ways) or they go party. Since Im taking care of my mom I can't be very social and Im not married or have kids as we have to share between, sister brother and myself to stay with mom when one is not around. Im over 40yrs old.

Anyway, for New Years Eve , no family get together like it happened in Christmas, this time each family member will spend it on their own with their own plans. Since this year my sister is in town I believed mom or me could spend it together it b e like the more natural thing, well that is what I assumed. Well no, my sister told me she does not have any plans for New Year Eve, because she does not feel like going out anywhere, she will stay at home with her hubby. Our brother will be out of town spending New Years with a male cousin and the cousin son (the same relatives of the Christmas gathering) at the beach. My cousin sister will spend it with her boyfriend at the boyfriend house, so my cousins parents will be alone at their house spending by themselves. My cousin parents can't go out cause my aunt is in a wheelchair and the husband does not drive anymore and much less at night.

So my sister said that since she is not doing anything why bother mom or me go to her house

Updates
6 mo
but also even if I arrange something at my house, she told me she does not feel like going out either even if she has a car. But also my sister says the weather be chilly and she does not want mom to go out and get sick either if we go to her house or my aunt house, as she is spending it alone too (her children wotn be with them either for New Year) ( I dont have a car).
Updates
6 mo
I feel my sister is selfish she is the one who can come to our house and spend he 4 of us together at the house but she does not want or feel like going out but instead she told me that inthe case we go to her house we have to take a taxi late at night to go back and forth and take care of mom for the cold weather.

Is my sister selfish towards our mom or family?
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