Sforno on Genesis 25:2/ספורנו על בראשית כ״ה:ב׳

1.ותלד לו את זמרן, she raised them in his house, similar to the “five sons of Michal” who raised the children of her sister Merav who had died at an early age, and had born those to Adriel (compare Samuel II 21,8) Michal had no children of her own at all. Although Avraham had personally only sired Ishmael and Yitzchok, he had adopted these children as seems clear from Chronicles I 28, and 32.