I am married to a Thai lady who has 2 teenage children. They want me to adopt them, (and they are old enough to decide) wife is also keen for me to adopt. I love the kids and have no reservations about attempting to become their legal dad. we have been together more than 5 years and bio dad hasnt been sighted for 12 years. Cant find anything on net other than adoption from orphanage, or other non parental scenario. Anyone have any clues, before I start battle with authorities.

Not good in that area myself but there has been a couple of pretty good posts about adoption here in the family section the last 6 months , search and see if they can give you some help. From what I recall, some posters (with first / second hand experience) seem to have knowledge close to what you need and have gone through or is now going though an adoption process. I suggest to PM them directly if they don't see this thread

Another option if adoption is not possible or too expensive could be to take the thing to juvenile court to legally get shared custody with the mother - the mother and the children (who clearly are old enough to have much to say in the matter) all agree so it shouldn't be too difficult. And also remove the fathers custody (if he has it), shouldn't be too difficult either since he abandoned them 12 years ago. Not all the way but a bit

Good Luck

One imortant question will be if it will be allowed under the rules of your own country. If you live in thailand, you can adopt under Thai law only. But that will in all likelyhood mean that the adoption will not be recognized by your own country. For that you will need to do an international adoption, according to the rules of Thai law and the law of your own country.

It goes to things as visa or not for your own country, nationality and inheritence in your own country.