Grannyjojo a member of my family has been in this very situation, and I'll describe how they have dealt with it, which has worked well. The in-laws supported their son quietly and avoided confrontations with the DIL. They continued to do the same things you have described, such as cleaning up dirty highchairs, ensuring the children had a clean change of clothes, and having the family round for good, nourishing meals when they could. DIL would sit reading in a corner, ignoring her children and family some of the time, shouting at the children at other times, but they didn't challenge her because she is quite a disturbed young woman who can get nasty and they didn't want to rock the boat for their son. In time, the son came to realise that this marriage had irretrievably broken down and he made arrangements to move to another home, which his children live in for part of the week. They now share the parenting and so far it has worked well for the children - it is early days, but the family are all much happier now. The son stayed longer in this unhappy marriage because he feared that she would withhold access to the children if he left, but this hasn't happened. She knows he copes better with the children than he does, and she likes her freedom.
Your son has good family support and his sister to confide in. If he is committed to the marriage, he needs that support to keep trying, but it might help for him to receive clear messages that you will all help him if he decides to leave, and for him to know he will have a home whilst he finds a place for himself and his children to stay when they are with him. You are acting out of fear because the threat of loss of contact with your grandson is unthinkable. That happened to me and my own son earlier this year and it was a very dark time for us. We fought in all sorts of ways to get contact back and for a couple of months we have now been seeing him regularly. Usually, bargaining with what we can offer in terms of financial help and practical support has swung it back in favour of us being able to see my grandson. I hope you are all able to find some resolution to this all too common situation. If only your DIL could accept your family's offer of a warm, loving relationship that includes her, but sadly some people really struggle to access what is on offer to them, and have just never learned how to have loving attachments to people they could trust. Best wishes x