Is a mother sinful for not visiting her own mother due to the injustice and problems she suffered from her and from her maternal uncles?

It is not permissible for your mother to cut ties with her mother, no matter how much she wrongs her. You should gently admonish her that this is a reprehensible act and that she risks incurring Allah's wrath. Upholding one's mother and obeying her are among the most obligatory duties, but it is not permissible to obey her if she commands the severing of kinship ties, even if the relatives are the wrongdoers. If you fear your mother's anger due to your maintaining ties with her relatives, then maintain these ties secretly without her knowledge, so that you may combine both rights. We advise you to strive to reconcile between them.