Well yoru seem to have an abusive mother ...on the other hand yamato's didnt seem like dat so i dont think they are gonna be brothers but half brothers or maybe distant cousin is a possibility...tho i do hope it doesnt end up like dat cuz incest is kinda no no
They're not blood-related and Yoru is taken in under the yakuza rather than the family register. So he's like an unofficial ward. The mom shown in the flashback is clearly not MC's mom who appeared a few pages AFTER showing the abusive one.
Yoru's mom: http://www.mangago.zone/read-manga/the_mountain_bird_cries_at_dawn/uu/iur_chapter-2/pg-2/
MC's mom: http://www.mangago.zone/read-manga/the_mountain_bird_cries_at_dawn/uu/iur_chapter-2/pg-4/
The yakuza leader is definitely not related to Yoru.
That's either his mom. Or I'm scared they might be brothers (〜￣△￣)〜