Well somewhere in all the posts up there you will see where we ran into a conundrum Mav.

You say "my mother-in-law's house in nice" because the whole term 'mother-in-law' is the noun BUT If mother-in-law is really a noun then why isn't it mother-in-laws (instead of mothers-in-law) when referring to the mom on both sides of the family. And is it mothers-in-law's houses or mother-in-laws' houses.

And when you refer to both parents on both sides it becomes an unweildy mouthful (leave that one alone dear - people are sensitive these days and after that whole Bingley size of fig-leaf debacle, well, nuff said)