I don't know that the kids will get sick; it gets pretty cold in air conditioning in the summer and just as cold in our winters as theirs :-)

If his mother feels she cannot come, then she probably cannot. I don't know that that is being selfish; I'm only 57 and yet beginning to understand about "old" and how it might be difficult for her to take a 4 hour bus ride from her home and then an 8 hour plane trip. Older people are not as resilient as young people nor as comfortable away from their home. I mostly feel badly for you having to take the brunt of the work.

I don't know about the 2 weeks back-to-back excuse your husband gives though; if it were my mother who is too elderly to come visit me, then I'd want to go see her no matter what; when else is he going to find the opportunity, when she dies? My stepson's in-laws live an expensive plane ride away and the grandchildren are only 3 and 2 but he flies down when my daughter-in-law does to help on the trip, if nothing else. He usually can only stay a weekend and sometimes, if my daughter-in-law is staying with the kids several weeks, he comes down the weekend they leave to help them come back.

Maybe you can suggest a larger family get-together? Is it his sister in Germany? He could call her back and suggest a date when your family and his sister's family could all go to Argentina to visit their mother? I think I wouldn't feel as resentful going to a larger party, even if I had to do a lot of the "work" if your husband at least went down with you and stayed a few days and some other family members were going to show up, etc.?