there will always be a reason or a holliday ....do what you need to do for you and your kids. if you feel as though he shouldnt be there or that he is not sober then saying no is completely justified as you know jumping through hoops as im sure you did in the past for him is stressful and can really dampen a good time.
if you belive him and think that this reunion for the hollidays is good for your children and i kind hand to your xah then thats reasonable also.
listen to your gut....if it fels wrong then it probably is