Why not decide that, as it is a lovely day today, you will have a treat and celebrate the start of spring and all go out this evening for supper. Even if it means that you take a picnic with you because you don't want to buy supper. Or have a macdonalds or go to a nice local pub or restuarant (lots of cheap deals for meals around at the moment if you look on the moneysavingexpert.com site - there's a list full of offers and vouchers).
Or go out to the park and come back late having picked up fish and chips en route or go to the local leisure centre to swim or play or whatever and have meal while you are at it. decide spontaneously and just do it. today.
what would happen if they arrived and you weren't there?
what would happen if you said you had been sitting down and working out your finances and realised how much of your weekly bill had gone on feeding them? And then presented them with a bill?
what would happen if your son didn't want to play with their children and you had arranged with him beforehand that he could watch his favourite video so they couldn't watch tv and had to entertain their kids - and you were to say to the parents when they arrived 'you don't mind do you, but little dc has had a hard day at school/needs a treat/has been a good kid/whatever and I have promised him that he can watch this on his own without any interruption from anyone else so sorry little nieces/nephews, no playing with xx today, you'll have to play with your mum and dad'.
what would happen if you got to their house just before they set off for yours and said right, we're coming over to supper here for the next couple of years?
do you think they moved close to you purely because you were close or would they have chosen the house anyway? Still, expecting to be fed by somebody for 2 years without giving anything back in return - surely they must realise that it is a bit strange and not on?
It's one thing to say that your sis will keep an eye out for your wife and kids while you are working away, have the odd coffee or meal at each other's houses. But to turn it into an open invite for every night - not on.
TOnight - if you don't go out - how about you just have a meal for yourselves and put a biscuit/slice of bread/apple in front of them and say sorry, I wasn't expecting you to come tonight...
good luck and let us know how you get on!