Thank you for using Just Answer. Sounds as if this is a very difficult situation. I can see your perspective and hers as well. But, the ones that may be harmed the most are the children. They probably do not understand what is happening since they have become accustomed to you, and now have a new female in the picture.
I think it will be very important to allow the children to have period of transition, rather than a quick halt of visitation and care giving from you. Once a month is not that often, and most grandparents do see their grand children much more often than this. If she is trying to acclimate them to her, they may instead see her as the one who is keeping you away, and fail to attach because of this.
I would strongly recommend you encourage your son and new wife to seek family counseling with you. For the benefit of the children, working together on this issue can prevent any emotional abandonment type of feelings in the children.
Please let me know if you have further questions. Thanks, Gina