What is the legal ruling regarding adoption and changing the surname in a country that relies on secular law incompatible with Sharia, and is it obligatory to file a lawsuit to annul the adoption even if it is rejected, and what is the fate of the parents who carried out the adoption?

May Allah increase your eagerness for good and forgive you. Adoption is forbidden in Islam. Islam nullified its effects with Allah's saying: {Nor has He made your adopted sons your [true] sons. That is [merely] your saying with your mouths. But Allah says the truth, and He guides to the [right] way. Call them by [the names of] their fathers; that is more just in the sight of Allah} [Al-Ahzab: 4-5].

Whoever practiced adoption out of ignorance of the religious ruling, we hope there is no sin upon them, and they should strive to nullify it officially if possible. If not possible, then they must strive to manage the matter to eliminate the religious prohibitions resulting from it, such as inheritance, mixing of lineages, and mahram relations, especially if your aunt has daughters. He should write a document to that effect and have witnesses.