While you are well meaning, you need to consider the effect on all of the children and your son if she were to relocate at a time when all are grieving the loss of their mother/wife. Your son needs her at home as a support to her younger siblings.
I know of several young women from Estonia and Russia who came to the university and have adapted well to life in the US. One now teaches at a private school in town, and the other is an instructor at the university. There really is no rush. It is better to do it correctly than haphazardly. If you apply for your son, he can then apply for his children. He can obtain a reentry permit in order to stay with them until they receive a visa number. This is actually a faster way for them to obtain green cards.
The key is for him to remain single until he has a green card otherwise his wait is much longer.
I still recommend you meet with an attorney; be sure they specialize in family (not employment) issues.

