Honestly, I've just read your second account of mil unreasonableness, and while there is poor communication, it doesn't seem to be all in their side.
Weeks before the event sil mentions it to you, under the impression you know. Easy mistake, maybe mil has forgotten to tell you, or maybe it's crossed wires and she thought sil was telling you. Your (and dh's) response is not to call up mil and ask about the event but to do nothing.
Three weeks later mil mentions event again, tells you the costs of hotel, you say you can't afford it. Fine.
The event is mentioned at least two more times by mil and sister who are still trying to facilitate you going and suggest maybe sil will put you up. You decide not to ask sil, or look into making other arrangements (like a hotel) and again you do nothing.
Finally you Dh 'eventually' checked the arrangements right before the event (something the two of you could have done earlier) to ask if sil can put you up and are told no. You then complain that you couldn't book a hotel because you didn't know where it was (did you bother to ask anyone? Or just sit and wait to be told?).
Basically the whole story comes across as mil and family trying to see if you want be involved, you and Dh not bothering to try to make it work or get involved in the planning, just sitting back and complaining that they don't communicate in the way you want. Then you complain that mil doesn't involve you the planning!
If I repeatedly mentioned a family event to someone who showed no interest in trying make it, I'd just assume you weren't that interested and drop it.
Seriously, take the initiative, you (or Dh) get involved and talk to people, and make the plans work for you too.