This all sounds rather daft to me. I can understand some people not wanting visitors to their new house until it's all done. We're in the process of buying a new house, and I already know that I prefer not to have people coming to see it until I have it just right with nice furnishings, etc. I won't be 'inviting' people to come over for dinner, but I certainly wouldn't stop my in-laws, or brother in-law from popping in to see it. Let's face it, we all like to see people's new homes, and to have to wait for more than a year to see your son's and DiL's new home seems a bit extreme.
I do feel that your DiL's reaction is a little petty, to ban her husband's brother from popping in to see the house, when he's offered to take you there to see it! If I were you, I'd definitely be speaking to my older son see ask what on earth the problem is. I wouldn't cause an argument, and I'd accept whatever reason he gives but, frankly, whatever the reason, he's more than likely to feel embarrassed about it, as it really does seem so trivial.
Either accept that you will have to drive yourselves, even though you don't enjoy driving. It depends how desperate you are to see the house. Or, simply accept what he says, and agree to visit once they've done all the work and settled in properly