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I am sorry that your son, you and the children are having to go through this. Unfortunately, with the mother and the biological father of the children both living, no court having determined either/both to be unfit parents, and there being a custody order already in place, he is not going to be able to get custody of those children. His claims end with the divorce, except that he may consider petitioning the Texas court to allow a continuing relationship between his child with the mother and her children since they are half siblings.
I have a stepson myself, who lived with us as he grew up, and can appreciate the love that grows up in these circumstances, but there is no legal basis for your son to have custody of children who are not his. Had the biological father been out of the picture and your son adopted the children it would be different, but as things stand, he has no grounds to gain custody or even visitation if opposed by the mother and/or biological father based upon the information in your post.
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