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It is certainly possible, but not something that can be legally compelled.
By this, I mean that he can make a request to his commander based on hardship. The commander can grant that request, but nothing in the law would require the commander to grant it because separations of this sort are done at a commander's discretion.
Your nephew should contact his chain of command, his chaplain and his commander directly to let them know about the circumstances and see if he can be separated, have his deployment postponed or some other action to allow him to see to his family's immediate needs.However, there is nothing he can do to compel a result. If his commander is not willing to budge on his deployment, he will have to make other family arrangements before he deploys. Most commanders are decent people and take care of their people, but sometimes there are conflicting interests that get in the way.
I wish your nephew well and how things can be worked out here.