gillymac, I can see why your Sil and intended would invite her parents to stay at their house for the wedding.
But, if I'm reading this correctly, it seems a bit off to have her other sister and family to stay, plus one of her friends, while not at least inviting her brother (your dh) and family, considering the distance you will be travelling.
Of course, I don't know what practical considerations have gone into this, or if your SIL has other siblings who are also making their own way to the wedding.
From what you say, however, the lack of an offer to stay at their large house would make me less inclined to pull out all the stops and go with dh. If you talk to your SIL about why dh is going alone, I'd be tempted to say in a nice but slightly pointed way that it was the accommodation costs that have pushed the weekend beyond your budget :0
AS for not going at all, thinking about it, I can see that a blatant lie might, as pupuce said, come back to haunt you. Your dh is her brother, after all. Sending dh alone is a good compromise. Is it any cheaper for dh to fly there? Could he do both journeys in a day, so no overnight accommodation is needed?