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I am sorry to hear about this situation.
1) You only mention one who is 12, and one who is 18. Is there a third child, or that is it?
2) Who has the 12 year old now?
3) Who has formal custody from the court for the children?
4) Is there a custody battle going on before the court right now, or not?
This is not an answer, but an information request. I need this information to answer your question. Please reply, so I can answer your question. Thank you in advance.