All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The husband of your daughter is not a Mahram to your second wife, and the latter is not permitted to expose her adornment in front of him, and she is obliged to wear Hijab in front of him. It is your first wife who becomes non-marriageable for him as soon as the marriage contract with her daughter is conducted as Allaah Says (what means): {…and your wives’ mothers…}[Quran 4:23]
For more benefit on the Mahaarim (plural of Mahram), please refer to Fatwa 85868.
Allaah Knows best.