Oh please. It takes two to tango. Chinese attitudes are much less toxic than the South Korean ones.
There is never a blood brother thing. Just ask the Slavs.
Koreans arent going to side with China for two reasons.
1.They are a vassal state
2.They have been in the shadow of China for almost its entire history and they see things reverting back to that.
You can't even say it's some Han Chinese westoid BS because the Mongols and Manchus did the same thing anyway. Korea is the subordinate and China is the boss. Koreans see many things reverting back to China and they will no longer get to stand out as the representative of the sinosphere. As Chinese gains more prestige, people will start asking questions on Korean stuff saying hey isn't that actually Chinese? You can already see it with Japan and those guys had way more time to create their own identity.
There's a reason Koreans hate China more than even Taiwanese who are supposed to be going to war with China soon. Many Taiwanese don't have this butthurt feeling since they know they are Chinese. If China wins, not much will actually change since many Taiwanese firms already get most of their business from China anyway.
But that's not really China's problem that Korea feels butthurt that it has identity issues. Ultimately it depends on who wins. Korea was noble in siding with the Ming over Qing but at the end of the day that was the wrong choice. And likewise if China wins over the west, you will see the same thing.