Yes, that is ok for them in their native country. However it is illegal in this country. What happens to the 13 year old bride?
Remember they are now over here.
That is still not illegal this country. Even if the State doesn't recognize the polygamous marriage, you still have de facto polygamy. There are many relatively trivial ways to work around this, and many do. For example, they will only be officially married (per US guidelines) to one wife, but they will still live with both (or depending on the culture, may live in different houses).
Arrangements like this are far more common (occasionally even among Euro-folk) than most people probably believe. You never hear about it because it is not technical polygamy only de facto polygamy, and therefore not a crime in most locales. And quite frankly, it disappears below the noise floor of normal American behavior in their nominally "monogamous" marriages.
The 13 yo will go back to her family or will be adopted by another family, probably her erstwhile husband's. These folks will not try to maintain polygamy. They are intent on becoming Americans.