I think you missed some important details. The reason the shaman is taking on revenge on Sosa (ML) is because Sosa killed his older brother, the one who warned ML that he should not interfere with MC any longer or MC's soul will be endanger. Sosa can't accept it and killed him. Unfortunately, the younger shaman witnessed his brother's death and that's the reason he wanted revenge.
Also, I think Sosa summoned the shamans to check on baby MC at that time so I don't think the shaman and MC are brothers. Why I strongly believe it? Is because the shaman also wants to kill the MC so that he can weakened the ML and successfully kill him.
You need to reread chapter 20 because your explanation is incorrect. Sosa didn’t kill the shaman’s older brother, he killed his father. Nowhere has it been mentioned that the shaman had an older brother, only that he has a younger brother (whose identity the OP is speculating about). The shaman has only expressed vengeance towards Sosa so far, not the MC.
Let's say they're "brothers" then and you want the shaman to "fuck-off" and "mind his own business" cuz YOU think the MC and ML should be lovey-dovey and heal together.
Remember, the shaman wanted to exact revenge on Sosa because he killed his father one-sidedly and kidnapped his younger brother which build resentment and vengeance. There's no way to stop the shaman's goal and bring justice for his family.
The author made the shaman look like the villain so he's kind of a plot device in the story to make it more interesting and meaningful.
I just wanted to point out what you said about "I think it would be best if he just fucks off" blah blah part.
That's all, bye
Dude? In chap 10, MC was almost got stabbed if not for the female snake protecting him, he also got hurt his back by a knife. The shaman deliberately fooled every head of the village including MC's, he said "It wasn't the wicked who were singled out... but the one who were disposable. And I felt confident that the boy... would be among those selected. After all, it was unlikely he'd fit in... while carrying such evil within him."
Got that?
I'm talking about the shaman's existence in the story and why he's in the story. I know you're frustrated why he's getting in the main couple's way but you just don't understand the character's purpose and meaning in the story. The author made him that way and there's more chapters to go. THAT'S WHAT I'M POINTING AT.

Just a theory guys, but I think mc and that other guy on last chapter r siblings in this life I think and, just saying I think it would be best if he just fucks off BC I think mc and ml just being together will heal each others sounds together and him trying to hunt(avenge, get revenge) is such a fucking sick move like just fuck off please,, but I mean I don't know his story but still ughh