OP here - thanks for the responses. I had my son write thank-you notes to the step-grandparents for his last birthday and Christmas, which his stepmom sent to them. We will do the same this year.

Mom definitely has other issues. She has had some tough breaks in life and has settled into patterns of constant self-pity and victimhood. I'm keeping my eye on things, because my grandmother's erratic behavior also escalated with the onset of dementia. She's very isolated as well, made worse by Covid, but also by the fact that she's alienated a number of longtime friends and family members in the past few years.

PP who asked about why I told her -- I didn't; DS did. At Thanksgiving, he was playing with one of the gifts they sent him (his bday was last month), and she asked where he got it. Things escalated from there, as they do with her. She has refused to apologize to my ex, so I did on his behalf. I don't believe he's mentioned it to his wife, and there's no way her folks would know.

Anyway, I've always been the one to extend the olive branch and smooth things over with her since I was a kid, and this is a good reminder that I need to continue setting and enforcing boundaries.

I agree that your mom is wrong about this.

But.

I think the fact the mother (OP) and stepmom have a good relationship is key here. OP seems to appreciate the SM and has no jealousy towards her nor her parents. This is the healthiest position for the child.

FWIW I am the position of stepgrandmother and I have tried to do the same thing for stepgrandkid. We live many states away so I am not at all infringing on biological grandmother's turf. My DH just sends checks but I have also included small gifts at birthdays, holidays and none of them have been acknowledged.

So I stopped.

OP. As *everyone* on this thread agrees, your mom is way out of line. You need to shut that down and tell her that she needs to apologize to your ex. I wouldn’t even want to leave my kid alone with her because who knows what kind of crazy and unhealthy ranting she would do about your ex, his wife or we parents. NOT okay for her to begrudge someone caring about your son.

OMG - I'm the step-aunt, and my whole side of the family showers my brother's step-kids with gifts the same as we are with all nieces and nephews.

What else is you mother ranting at your Ex about? He needs to block her and her selfish, poisonous attitude. She wants to have your DS all to herself when he could only benefit from having more supportive adults in his life. Her jealousy is so unbecoming.