So why is the son-in-law not being punished too? Or not just yet seen it

Probably because his daughter chose him and he's afraid that she'd be sad if he got punished.
I personally would love to give a punishment, but everyone's built different ig

https://www.mangago.zone/read-manga/to_my_husband_s_mistress/uu/nml_chapter-22/pg-1/

Chapter 22 makes it look like he wouldn't I bet she gonna divorce him

As she should. Throw that man into the streets