Why does it say above (verse 5) "l'Yakov Ben", and here "Ben Sheni l'Yakov"?
Malbim: The bigger Chidush is taught first. Above, the Chidush was that the son was attributed to Yakov. Now, this was already known; the Chidush is that she had a second son. Also for Zilpah's son, it says "l'Yakov Ben", and "Ben Sheni l'Yakov" (verses 10, 12). 1
Targum Yonason says that Bilhah and Zilpah were freed before Yakov married them. If so, why is Bilhah called "Shifchas Rachel"?
Ha'Emek Davar (3,7): This shows that she was unlike Hagar, who [after she became pregnant] belittled Sarah. Bilhah continued to [respect and] serve her, like her Shifchah (even though she was freed).