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I will try to best answer your question.
I would not advise allowing your child to live with mom unless absolutely unavoidable. Mom will then be able to claim your oldest for child support an offset against her obligation for your other child. I would strongly suggest trying to encourage your oldest to wait until 18 before living with mother. That way Custody order modifications can be avoided an only child support will need to be dealt with. I'm concerned also that the move is in her best interest as opposed to mother pushing for it. I am making this assumption based on the fact that your youngest still wants to reside with you and this change of custody for your oldest is also going to separate the children. My advice would be to try to delay and avoid changing custody. Don't say no but come up with excuses and reasons to delay. For example you can say lets try having you spend a large part of the summer with mom and see how that goes before we make a permanent change. That would buy you about 6 more months of custodial time. When you reach the summer just agree to some summer visitation. After the summer visitation you can then say you might as well wait until you're 18 as its not far away. Once she is 18 then I would try to negotiate the termination of the support if she moves to Texas as that state is required to end at age 18.
I hope this information and advice is helpful and I wish you the best of luck.