LOOOOOOOOOOOOL you should ask Kishi cos that's hard to answer.
However, it is highly plausible that SO6P's brother will appear if Kaguya appears.
I think he might have used his life to seal Kaguya in the Shinju Tree, since Hagoromo and his brother both sealed Juubi and the God tree; knowing that Kaguya's essence was in the Tree, the brother must have done something.
Also, i think Hagoromo is the related to Uchiha and Senju, while his brother is more related to Byakugan and Uzumaki, So definitely he must have used a seal to ensure that once Kaguya is out, he (being the ultimate sealer) will also appear to try and reseal her again. And Kishi has shown us this through Naruto's 9 tails seal and the fourth Hokage
This is my answer to you bro