My D-I-L of over 10 years, is the same with us, she is 6years older than our son, they have a daughter of 9. Son phones us daily usually if going to the shop, if we go to their house (arranged beforehand) and she is in the house, she stays upstairs, perhaps coming down just as we are ready for leaving !! We have got used to the situation now, but still feel left out.
It was her birthday a few weeks ago, I sent her a card, signed it, and put in brackets under our names "the outlaws", I asked son what she thought of it and he said she laughed, the sarcasm I intended probably didn't get through to her though, but I felt better for putting it in writing.