Man, that's a HUGE family! LOL I guess the more, the merrier in this case.
I would say that it would be proper etiquette to invite everyone who's already invited to the wedding to the AHR. Also, if you include that there will be an AHR upon return, some people may decide this is the better option for them. So this may cut your guest list down considerably. You never know tho with DWs...
Good luck, and let us know how it goes!