The smart assed side of my would say "I'm sorry I can't accept you for company parents, but I have a dental appointment, so you'll have to come around some other time."  But the side of me who is EXACTLY like you, I say this because I live away from my parents too, and bend over backwards to accomodate my family all the time (and it's never reciprocated) say to you "keep on keeping on".  It's hard to accept when someone does not invest the emotion you put into to your kids and much less, the people who raised you aren't helping you out on the few occasions that they could.  It makes your blood boil!!! 

     You are different stock than those you have mentioned in your family, don't change being a beautiful giving person because of them.  You'll sleep well with yourself at night knowing your always doing the right thing, on the other hand you don't have to be a push over when the come to visit next week....... I think cool and aloof would be appropriate.

Best of luck with your situation, know there is someone out there who is in your shoes and I feel for you.....