When is it okay to talk back to your SO's mother?

My fiance and I have a 17 month old son. I have posted previously about how my SOs mother barely comes around, has only seen our son maybe 10 times and she only lives a mile down the road. My fiance always calls her and leaves her messages. She almost never answers or returns his phone calls and always makes excuses as to why she can't get together for her to see her grandson.

As of lately she has been trying to make my fiance and I feel bad for our son not knowing who she is. The last time I saw her which was almost three months ago she hugged me and cried and asked me not to keep my son from her anymore. I told her that I dont keep him from her and shes more than welcome to come by any time because one of us is always home.

My fiance talked to her last week to tell her that we are expecting another little one and apparently she made a comment saying hopefully we dont keep this one away from her too. My fiance has told her multiple times that we dont keep him from her and that he calls her and she never returns his calls and never stops by. She didnt even call my SO back today when he tried calling her for mothers day 🤦‍♀️

It's getting the point where I'm fed up with her trying to make me and my fiance feel bad and responsible for our son not knowing who she is and for supposedly keeping our son from her. I feel like if she says something to him or myself about it that I may just tell her off. We shouldn't have to chase her and beg her to be a part of her grandsons life and she needs to stop putting that blame on us. But at the same time I dont think it's my place to say all that much to her and disrespect her. I feel as though that's a conversation my fiance needs to have with his mother AGAIN. Even though it's just like beating a dead horse. Maybe it's just the pregnancy hormones but I'm getting annoyed, frustrated and it honestly hurts my heart for my son that hes missing out on a grandmother.