I agree with Terri... IF hes INTO you .. he will not be entertaining the ex in any way .. shape or form...
He will go to thanksgiving dinner.. and if he has planned to catch up with you over your days off.. (when hes not working) he SHOULD still do that...

I dont mean to be negative here.. but Lynne are you really sure hes over and done with HER???
I understand that his sister is possibly stirring the pot.. BUT I can tell you .. no one in my family would dare do anything like that to me.. that is not showing him any respect in the least .. that is why I ask is he really done with the ex... maybe he is talking a different talk with his sister than he is with you ....
Even though you have a 'casual' relationship Im sure you would want complete honesty from him. You deserve that.

SOOOOO ... do you feel threatened by him spending time with the ex??? thats the question you need to ask yourself and answer yourself honestly..
IF you trust him completely ... then there is nothing to be worried about ..

hope you have a good thanksgiving regardless..

huggs from ME

Take  what  you  need . . .  leave  the  rest .Blue_butterfly