Elias saved Estrella’s mom from persecution and nobles pushed for Elias to take a concubine to weaken the bond between Elias and Selina.
Selina actually approves of Estrella’s mom bc 1) by the time Elias brought her to the palace, Jeren (oldest son) was already born and there wouldn’t be a fight over successor 2) Selina and her actually became good friends 3) Estrella’s mom secluded herself only wanting a simple life
Estrella’s mom only got pregnant bc of the consummation of the wedding.