ציטירנדיקער פּירוש אויף בראשית 30:5

The most convincing proof that we are correct is found in Genesis 21,13 where G'd promises that the son of the slave-woman (Hagar) will develop into a nation. It is important to note that the Torah does not refer to Ishmael as Abraham's son but as Hagar's son. Thus the Torah emphasises that Ishmael remains a slave, seeing that he is the son of a slave. If you wanted to see further proof of this look at the way the Torah describes the sons of Zilpah i.e. Leah's slave-woman, and the sons of Bilhah, i.e. Rachel's slave-woman. You will observe that when the Torah described those women giving birth (Genesis 30,5) that the sons are attributed to Jacob, not to their respective mothers. This means that because Jacob slept with them in order to establish a legal bond they became free women. In the reports of the births of all the four sons of Zilpah and Bilhah the Torah was careful to emphasise this point, (compare 30,7; 30,10; 30,12) In our case the Torah empahasises the difference in status between Ishmael and Isaac once more when we read: "for through Isaac your seed will be known" (Genesis 21,12) i.e. not through Ishmael.

כי גם זה FOR THIS ALSO — also means additional to you, over and above Joseph. Our Rabbis explained that with each of Jacob’s sons a twin-sister was born, whilst with Benjamin an additional twin sister was born (Genesis Rabbah 82:8).