Thank you for your very moving question. I am particularly moved by the reactions of the sister-in-law.
I to have been around Hispanic women for more than 18 years...try 30 years. I have been in long term relationships with them (5 years or more) on three occasions, nearly marrying one woman, from Peru. I continue to this day to have close friendships with these Hispanic women, and the Peruvian and I have been friends for 30 years.
The tradition of the Kitchen, as I like to call it, is largely familial, and changes slightly from country to country. When I say familial, I mean that some families in each country hold to the traditions, while other's modernize and share the kitchen with the men of the family. I know a Chilean couple, in chili, and a Peruvian family, in Lima, and I see the men in the kitchen, from time to time, except during times of when they are entertaining guests. So in situations at least where all those person would be together, the woman does in fact cook for all.
But, I have also seen where because of some slight, or issue with the others, the woman may protest and not cook. She goes on strike so to speak.
AND she would get the support of the other women if they were justified, at least in the Chili, Peru and Puerto Rico.
I do not think there is a true bias here. It sounds to me like, she has been treated badly in some way by the others; or it could be that they disapprove of the persons she will not cook for, for some reason.
If you can find out what has happened to cause this treatment, then you can begin to help to fix it.
I have a Puerto Rican Friend, who has not spoken to her own brothers, for more than 10 years, because of slights that occurred at her mother's funeral. At a recent funeral event or another relative, they showed, up, and she refused to cook for them, but she cooked for the others.
I know a Peruvian Mother-in-law, who did not approve of a marriage, and she refuses to serve HIM when they come to the house.
In my cross cultural experience and studies, I have seen that in traditional Hispanic households, within their home land environments. women do not hold the same level of control and power as men. In those households, some of the so called "power" the women have, is expressed in not sharing feelings, thoughts, and kitchen duties. So they will refuse to talk, or refuse service...refuse to cook, etc.
So in your instance it appears, that if it is any bias at all, it would be that for some reason she does not approve of the in-laws; or there has been some injury..some slight.
Find out what that is, and you will have the key to helping them to reconcile.