Sounds like your mother is having trouble wrapping her head around the whole issue, and, because your sister is pregnant and living at home, she is the focus and priorty for now.
Have you thought about meeting your parents somewhere for a visit, or inviting them to your place for dinner or lunch?
As for Thanksgiving, if you are not comfortable I would pass. On the other hand, if you want to go, I would, if your sister doesn't like it...Oh well!
Whatever you decide, this situation is not going to be resolved any time soon, you may want to consider detaching a bit.