Wow. I have so many different thoughts about what you are going through. First of all, I want to say that I'm impressed you haven't had a meltdown and punched her in the face. It would be SO hard not to.
Secondly, you are quite the good wife standing by your man. I'm glad he gives you support behind mommy's back, but he needs to make it clear to her in no uncertain terms that she will no longer say ANYTHING negative, do ANYTHING negative or mean, nasty, degrading, belittling or generally disrespectful EVER AGAIN. He needs to tell her this
while you are standing there.
He needs to tell her that even though he loves her, he ALSO loves you, and she is not to disrespect you or his family. The consequence for doing so must be harsh. She loves money, so
hit her where it hurts the most. Have him tell her that he spoke to a lawyer and that he will be getting his fair share of the proceeds from the property including all profits EVER MADE. Then she will have to pay your family. And, if she continues her awful behaviour, he will make sure she gets put into a care facility for people who are not of sound mind (alzheimer's or dementia, or perhaps alcoholism/pills rehab center).
Wonder what she would do if faced with that?
My bet is you will never ever get the respect you deserve from this woman. She will continue treating you the way you have allowed her to treat you for the past 4 years. I know you did it out of respect and love for your husband, but she will actually respect you more if YOU stand up to her.
Third, I'm very sorry for the loss of you hen Joy. I haven't experienced that yet.
Fourth, your son is old enough to understand what is going on. Sit him down and tell him you love grandmother and that she is mentally ill. That might help him process why mommy puts up with foul treatment.
Fifth, GET A JOB off of the property. Earn some money for yourself. At the very least, you will have money for a holiday, and perhaps a way to buy more chicken feed or other things needed for the farm. You may find that being around other people will help you feel more normal.
Hang in there and know that we are all pulling for you, keeping you & your son in our prayers.