why wasnt the sage and his brother immortal just like kaguya i mean they were her children and their chakra came from the fruit that she ate so why didnt they live as long
Because her power was divided between them in the same manner Hagoromo's was between Indra and Ashura. You need the full power of the Shinju to be immortal
Because Kaguya possessed the power of the Shinju, her children didn't.
Madara prior to absorbing the Shinju nearly died against Gai. After absorbing it made him assert that he was invincible and impelled Naruto/Sasuke to rely on Fuinjutsu.