Wa `Alaykum As-Salamu Wa-ramatullah Wa-barakatuh

Islam allowed people to marry more than one wife with a condition of justice by time, care and financial support. From what you have described, it looks like more of a sexual need from your husband rather than a true responsible approach of starting a new family. Adding to that the haram way they are interacting with each other, I do not think this is the proper way to start a new family.

 

I also feel that he might need help and support from you to act properly. Stay next to him for the sake of the family and kids but with a condition.  He must stop acting in a haram way with her and be unjust to you and your kids.

Almighty Allah knows best.