Is a mother sinful for not attending her niece's wedding, on the grounds that social custom dictates waiting 40 days after her maternal uncle's death?

A woman is not permitted to mourn for a deceased person for more than three nights, except for her husband, for whom she mourns for four months and ten days, based on the hadith (definition) of Umm Habibah, which is agreed upon. Furthermore, showing grief for the deceased for more than three days is not appropriate. Therefore, your mother's refusal to attend her cousin's wedding due to mourning her brother is not appropriate. She should accept Allah's decree and prioritize Allah's pleasure over people's pleasure.