This is going to be a hugely important day for your step-daughter hosting Thanksgiving in her new home. She's not trying to screw anyone over, she's just stepping up to this special role of hosting Thanksgiving.
Right now the thing she's worried about the most isn't the turkey going wrong, it's how her mother and step-mother are going to get along. But she can't not invite either one because they are too important, and the fallout from not inviting someone would be extreme to say the least.
When you go... and you must go, because not attending is going to be a huge snubbing insult to your step-daughter, go with grace and dignity. It's your step-daughters show, so follow her lead and ask if she needs help with anything. Help out happily even if the task is below your usual skill set and responsiblity for a Thanksgiving dinner. Set the table, make the salad. Whatever.
And find something in all this to be... well you know... thankful for.
And no harm in dressing better/hotter than the ex-wife... just don't draw attention to it lol.