Nice way to insult the family.
You should either invite all the cousins, or none of them (except for situations like "cousin #1 is like a sister to me, cousin #2 won't give me the time of day").
As for your sister, it's really up to her whether she goes or not. In her shoes, I probably wouldn't go, but that's just me.
Next Thanksgiving, invite everyone in the family except her. Tell her you didn't invite her because you only wanted one turkey at the table.
