Wow, this is so easy.
1. Get the paternity test done.
2. Get the Court involved. Your DIL is legally obligated to provide everything for her son until he's 18.
3. I'm not sure how you ended up with any of your son's estate, even if he died intestate, the Probate Court wouldn't have awarded you any of it when he has left a widow and an heir.
If he hadn't made a new Will since he married, the old one would have become defunct merely by his taking a wife.
But, regardless, put those assets into a Trust for your grandson's tertiary education - that way it can be used for university or trade school, or even the last bit of high school if your DIL is particularly immoral and kicks her offspring out on his 18th birthday, rather than waiting til he completes his bachelor's or trade certification.
I Refuse to Let My DIL Get My Late Son’s Inheritance

Family inheritance can become one of the most emotional and divisive topics, especially after the loss of a loved one. Questions about who deserves to receive money or property often stir up tension, guilt, and resentment between relatives. Recently, one of our readers shared her story about facing this exact situation after her son’s passing, when his widow demanded access to his inheritance.
Diane’s letter:
Dear Bright Side,
My 32 y.o. son died just 3 months ago in a car accident.
His wife of 8 years has already found a new man and is relocating to New York with him. She’s taking my 8-year-old grandson with her and demanding my son’s $90K inheritance from me.
I said, “You don’t deserve a dime! That money is my right! And my grandson will receive his share when he turns 18.” She smiled.
But then I froze when she looked me in the eye and said, “You will not like how this will end.”
The next day, my grandson came crying. I froze when he revealed that his mother had decided to move to New York with her new boyfriend—without him.
I called her, and she said, “Well, since you decided not to give me a dime of that money, it’s on you to raise your grandson until he turns 18.”
Now my whole life is turned upside down. I am 65, and even though I love my grandson with all my heart, I’m not in the physical or financial condition to take full care of him.
But I still believe that my DIL should not get my son’s inheritance. I don’t want her to blow it on another man after moving on so quickly, as if my son never even existed.
What should I do?
Sincerely,
Diane

Thank you, Diane, for sharing such a painful and complex story. Losing a child is unimaginable, and now you’re facing both grief and responsibility at once.
Here is our advice to you:
Clarify Legal Ownership Before Acting.

If there is no will the spouse gets everything hate to say it that's the way it is Grandma is angry because her spouse has moved on it's none of her business and yes when someone dies you can throw them away Grandma can go screw off
Your son’s inheritance may not be as straightforward as you think. Consult a lawyer to determine whether the $90K legally belongs to your son’s estate, your grandson, or you. If it’s your son’s estate, your DIL could have legal grounds to claim a portion as his spouse.
Clear facts prevent emotional decisions that could backfire and ensure your grandson’s future is protected lawfully.
Negotiate a Conditional Agreement With Your DIL.
Instead of flatly refusing her, consider setting strict conditions if she wants access to any of the inheritance.
Propose a written agreement that part of the money goes toward your grandson’s needs only — like school, healthcare, or housing — with receipts or joint oversight. It gives her some access while ensuring your son’s legacy supports his child, not her new relationship.
Protect Yourself From Being Overwhelmed.
Caring for a child at 65 is emotionally and financially demanding. Apply for guardianship assistance or social services in your area. If his mother left voluntarily, she still has a legal obligation to contribute financially — you can pursue child support through family court.
You’re doing your best, but you shouldn’t carry this burden alone. Legal and state support exist for exactly these situations.
Give Your Grandson Stability, Not Sides.

He’s already lost his father and now feels abandoned by his mother. What he needs most is emotional safety. Avoid speaking harshly about his mom, even if you’re angry. Focus on routines, reassurance, and therapy if possible.
Protecting his emotional health now will help him grow up secure, and eventually, he’ll see for himself who truly stood by him.
Another story that’s sparked a lot of discussion comes from Gemma, a vegan who invited her friends to celebrate her birthday at a plant-based restaurant. But when the bill arrived, her friends refused to pay for their meals. Do you think they were right to do that?
Comments
Make sure to file with cops about fact DIL moving to NY without her minor child, this could abandonment, even if left with Grandma. DIL punishing her son because MIL won't fund her life with her new BF. Makes me wonder if had been cheating on OPs son of already had new BF 3 months after his death and living together
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