Guardian rights for those in another country

So I'm doing research for this book I'm writing, and in it the main character finds out she was born via double donor conception, and after some work she finds out that her donor parents both are alive and living in England (whereas the main character lives in Minato with her parents and brother). As such she decides to contact them both and ends up becoming close with them both. In the book through some friends the donor parents had gotten into a relationship and had married prior to the book (but after they had both separately donated).

At this moment I was wondering if there is any legal way for the mother of the main character to petition to give guardianship rights to the donor parents and/or put them as the ones to adopt her in case of the parents being in an accident of any kind as the mother in my book is adamant on allowing the main character to grow their relationship with the donor parents?

Uhhhh... What???

I don't get the point of your question. If this is for a book, why not contact people on a forum that's related to writing? Or are you asking a question about actual legality of things and just using the guise of 'I'm writing a book and this is my character' when it's really about you?

Either way, no idea what you mean and if you have legal questions, ask a lawyer.

It depends on what country's law you're talking about.

Are you saying that the donors are UK citizens and that the legal parents flew to the UK to obtain their child? Where is Minato? A fictional place in Japan, perhaps? Why are you posting on a forum related to Japan?

For example, if you want UK citizens to adopt a child, you need to look into UK laws. As for laws in Japan, here are some details from the Ministry.
https://www.moj.go.jp/MINJI/kazoku/youshi.html

in foreign countries, do parents have a "right of guardian" ?
I think, in Japan, parents have duties or responsibilities for their children under the name of guardian. this duty (or responsibility) are not transferable. when they can't perform their duties by some reasons, a family court, a third organization, may take their duties out from the parents and legally order someone as a guardian.

what parents can do is either one, performing their duties or not performing their duties (abandoning their children).