Families often feel emotional at holiday time especially if a parent is unable to join them to celebrate.
You and your sister seem to be aware and respectful of your mother's needs and comfort and that is to be commended. You are doing her a service by not overwhelming her in a crowd.
If it is possible, you can celebrate with her in a different setting and at a different time. Most facilities plan holiday celebrations for residents at a time different from the specific holiday.
This enables families to come there and dine with their loved ones rather than taking them out of their "comfort zone." If her memory support unit does not offer this, then you can plan your own "mock Thanksgiving Dinner" with her. It is common for memory-impaired persons to be disoriented to day/time so you will be creating a Thanksgiving Day of your own.
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