My first piece of advice is for your sister to stop correcting her mother in law - cold turkey.

A daughter in law's conversation with her mother in law should be strictly limited to compliments.

 

Secondly, both halachik scenarios you described may have specific heterim depending on various particulars. Perhaps a review of the halachos with a knowledgeable Rav may help your sister determine what is considered a chumra or a minhag rather than a full blown issur . Don't allow a chumra get in the way of a perfectly fine relationship, or worse, a perfectly fine cholent

 

My advice in this particular circumstance would be for your sister to talk to her husband - the son; explain to him what she has witnessed and see if he can straighten it out. This will of course depend upon the son's relationship with his Mom.

 

Better yet, I would have the son, her husband, talk to his father and have his father point out the various halachos to his wife.

 

If this does not clear up the situation I would advise both your sister and your brother in law to seek a knowledgeable and experienced Rav TOGETHER and discuss the particulars with him. The Rav should be able to advise the couple as to what the Torah requires of them.

 

Hope this helps!

 

Regards,

 

Tzvi Frank