Why do my sisters want to live in the USA instead of living here in Mexico?

Have you asked your sisters? For your married sister, I'm sure that she wants to be with her husband, who I'm guessing is American? There obviously is a pull for them to live in the US. Perhaps because it's a different culture, and could be interesting to them. Whatever the case, just respect their decision and be glad for them if they are happy. Besides, the US is NOT such a bad place to live. If Mexico was fantastic for all, there would not be so many leaving illegally each year.