I am so sorry you are experiencing this. Your DIL is an emotional abuser because her focus is on “I” and what can you do for me, always. Even though she is married to your son, they are not truly a couple because mutuality and equality are absent from the relationship.
I have a daughter who is a manipulator and user. I found Al Anon tremendously helpful in learning how to deal with her and to set boundaries in our relationship. Though my daughter isn’t an addict, the manipulation that is present with addicts is something I needed help with. As with any drug or alcohol abuse, treat the situation like she has a disease. There has to be some compassion present because she is a sick person.
Here is what I have learned from my two years in Al Anon:
Don’t hate the addict, hate the disease.
Don’t hate the person, hate the behaviour.
If it is hard to watch it, imagine how hard it is live it.
Learn and practice detachment, which is removing yourself from someone else’s outcome. Detachment isn’t selfish, it’s caring for yourself and letting others care for themselves.
Trust your intuition. You don’t need to explain or justify your feelings to anyone.
“No” is a complete sentence.
If your son quits his job and asks for monetary help, try detachment. It may sound like this: “You’re young, smart and resourceful. I’m sure you can figure something out.” Then walk away.
Through pain comes growth. It sounds like they both have some growing to do. I can highly recommend Melody Beattie’s book Codependent No More.
Christmas is just another chance for DIL to use you. She expects you to hold a separate, special dinner because Christmas is reserved for her family? Maybe try, “sorry we’ll miss you this Christmas, hopefully, we’ll get to see you next Christmas”. Christmas gifts only come on Christmas, so if they don’t show up, then neither do the gifts. You can give your grandson a gift the next time you see him.
All said, you still need to try to have a cordial relationship with DIL. Listen politely and very carefully to what she says, and then go and do exactly as you please. Come to terms with this: she may not be the daughter-in-law you dreamed of, but she is the daughter-in-law that you have. Choose to have whatever relationship is possible with her, for the sake of keeping contact with your son and grandson. If your DIL sees you cannot be ruffled, she'll have to change her act and smarten up over time. Put the grandson and son first and be as lovely as you can, leaving the venting for friends or your hubby.