This question explains How to send pull request from my fork to another fork?
However, it only works if both forks are children of the same parent fork.
Now, I have a parent fork, and I want to send a pull-request to a child fork (the opposite of the usual direction). The problem is, I do not see any "Pull Request" button on my fork. What can I do?
When you’re creating a Pull Request on GitHub, you can change which way it goes. Make your fork the Base Repository.