Short answer: No. Your question seems to be based on a misunderstanding of Texas law. It is true that if this father has two other children to support, his child support obligation to your grandson would be decreased to 16% of his net income (from what would ordinarily be 20% of his net income if he did not have these two other children to support). However, the fact that the father has two other children to support holds true whether he is divorced from their mother or not. In other words, the Texas court will customarily apply the lower 16% guideline regardless of the father's marital status. This is because he has a legal duty to support these other two children whether he is married to their mother or not -- either way, these two children are costing this father money each month to support them, and the Texas court is going to take that into account by using the 16% figure instead of the 20% figure. So, the notion that the amount of child support to your grandson will change by virtue of the father's divorce is simply incorrect. Either this father is simply mistaken about the effect of his action to divorce his wife, or he must be divorcing for other reasons (for example, perhaps his wife just learned that he fathered a child with another woman during the marriage). If she can afford it, your daughter should have an attorney.
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