I really do send you my heartfelt sympathies, it must feel similar to a bereavement.
Families cause an immense amount of stress. You don't give too much background, but is there a culture clash here, is your DiL Chinese? The other worry that occurred to me was whether she was suffering from anorexia or bulimia, if she wouldn't eat with you? Could she be seriously depressed?
Did you not discuss the child care arrangements as soon as she started staying in her room, it seems very strange behaviour, I would have wanted to know the reason, you would think her child would seek her out? What did your son think about this, he seems to have gone along with it, which leads me to think that she must be ill in some way.
As you say, there must be more to it, but I think I would just text your son saying you are worried about all three of them, and you are here for them if they need any help in any way.
Relationships with DiLs will always have an element of treading on eggshells, some much more than others, all you can do is bite back any criticism and keep communication open.
And please look after yourself too! 