Obsidian is not a fork of anything else. We wrote and own the entirety of the source code of Obsidian, with the exception of all third-party libraries such as Electron, Mermaid, MathJax, and CodeMirror.
SiYuan’s authors started working on their app in August 2020 after hearing about Obsidian’s success. SiYuan is close sourced by the way. The authors turned their previously semi-abandoned open-sourced project called “LianDi”, renamed it to SiYuan, and close-sourced it when it was released. They exclusively target Chinese audiences (their website requires a chinese phone number to register).
I’ve had the chance to have a few discussions with the author of SiYuan, and the reason why SiYuan and Obsidian are so similar is because they have recruited most of their users from a large Obsidian QQ discussion group (Chinese group-chat platform). It is only natural that these users demand features currently in Obsidian, thus pushing SiYuan to replicate similar features in their app.