I'd have to agree that its OS's "job" to speak to YS, or any other employee that has issues needing addressed. And it may be a good learning experience for him as well.
No sense letting the codie grow, huh? ; ) The poor guy is probably stressing over his brothers behavior, AND the impact on his reputation as a boss.
BUT...
I know me and I'd probably open my big mouth at some point, hoping that a word to the wise would give my YS options, if I felt he was unaware of whats coming.
Maybe you could stay out of the issue, but still talk to YS about career options, perhaps something he feels he would excel at?
Seems having them work togather will just drag Mom in time and time again.
Whatever you decide, good luck!
((((HUgs))))
Cece