Thanos, the Mad Titan, adopted both Gamora and Nebula as part of the back story in Guardians of the Galaxy. In a powerful demonstration of how NOT to act in the best interests of a child, Thanos adopted both children after killing their families.
In further proof of telling a child “I love you just the way you are,” both Gamora and Nebula were physically altered, including bionic implants in Nebula. Both were taught how to be assassins, including the use of swords and firearms.
Great example on how NOT to be Dad of the Year.
“Adoption” is the creation of a parent-child relationship by two parties who usually are unrelated. Adoption legally creates all of the rights of being a parent and child, from the responsibilities of a parent and intestate succession. See, Black’s Law Dictionary App, 9th Edition. Each state has its own set of laws on adoption.
There is no way any judge on Earth would have permitted the adoption of either Gamora or Nebula by Thanos. The giant reason is Thanos killed the families of both women while they were children. While it was true that both young women were orphans, no judge will give custody to the person who murdered the biological parents.
Granted, the only reason Thanos was physically successful in adopting Gamora and Nebula was he destroyed their worlds and respective legal systems.
Generally speaking, someone is suitable to adopt a child if: 1) They are at least 10 years older than the child; 2) Will treat the child as their own; 3) Will support and care for the child; 4) Has a suitable home for the child; 5) Agrees to adopt the child. (See, California ADOPT-200 Adoption Request and CA Family Code 8600-8622).
No Court Investigator will find a floating asteroid as a suitable home for a child. Moreover, living on an asteroid likely does not have the support and care a child needs for proper development, to say nothing of socialization. Furthermore, subjecting a child to bionic implants and weapons training could in no way be in their best interests. The bionic implants would be a form of child abuse through unnecessary medial experiments. The weapons training would child endangerment where a child could actually lose an eye.
Thanos would quickly be rejected as suitable parent to adopt any child. The denial of his adoption application would likely result in the destruction of the courthouse, so best to have the Avengers, Adam Warlock, Captain Mar-Vel, and SHIELD on hand. It would also be wise to have the children appear remotely for the hearing, so they would be safe in the event of violence.