Basically you are asking about: Certifying custody in Thai law for parents who are not married.
These are the relevant legal articles:
Section 1546. A child born of a woman who is not married to a man is deemed to be the legitimate child of such woman. Section 1547. A child born of the parents who are not married to each other is legitimate by the subsequent marriage of the parents, or by the registration by the father, or by the registration made on application by the father, or by a judgment of the court. In this section 1547, a child born of the parents who are not married to each other will be able to be their child by law in 3 cases are:
1. Parents are married later.
2. The father has registered to certify that child is his child (with consent of the mother and child)
3. The court has ruled that a child is his child. (By filing the case in a lawsuit)
Section 1548. When legitimation is applied for by the father, the child and the mother must give consent to the applicant. In this case where the child and the mother do not appear before the Registrar for giving the consent, the Registrar shall notify the child and the mother of the father’s application for registration. If the child or the mother raises no objection or does not give the consent within sixty days after the acceptance of the notification by the child or mother, it is presumed that the child or the mother does not give consent. The period of time shall be extended to one hundred and eighty days in case where the child does not reside in Thailand.
In case where the child or the mother raises an objection that the applicant is not the father, or does not give the consent, or is unable to give the consent, the registration must be effected by a judgment of the court. After the court had pronounced a judgment in regard to the registration of the legitimation and the judgment has been produced to the registration, the Registrar shall put the registration in effect.
That is what the law specifies. In your story, it is clear that you are not married to the mother, so the child is deemed to be legitimately the mother's child. So as a first step, you are required to go to registrar office to begin an application of registration your daughter, which will need the consent of your ex-partner and your daughter to have you become the legitimate father. If either the mother or the daughter don’t give consent, then you need to go to the court and sue them to get certified as the father of your daughter in juvenile court. The court will then rule on your case to acknowledge your parenthood of this child. After that, you need to take the court’s order back to the Registrar office to continue your application.
You also mention that your ex- partner has no interest in being mother and is more considered as a sister of your daughter. This opens the door to sue her to try and revoke her custody of your daughter. You do need to provide ample proof and documentation of this situation, which may be quite difficult. You could follow the procedure above and have this situation mentioned, to create a formal record of your concerns for potential future reference.
Please feel free to send us a private message if you wish to discuss further.
Troi Family Lawyers