Is it permissible to attend a wedding that is held at the same time as the third day of the passing of the cousin's daughter, taking into account the psychological need of the deceased's children for the questioner's presence?

There is no harm in attending the wedding, and it should not be avoided due to grief over the maternal cousin's daughter. Islamic law has prohibited showing excessive grief for the deceased for more than three days, with the exception of the wife. Al-Bukhari and Muslim narrated from Umm Habiba, may Allah be pleased with her, that she said: I heard the Messenger of Allah, peace and blessings be upon him, say from the minbar: "It is not lawful for a woman who believes in Allah and the Last Day to mourn for a deceased person for more than three days, except for her husband, for four months and ten days." If there is a need not to attend the wedding, such as fulfilling the request of the deceased's child, then one should strive to reconcile between the two matters, by visiting him to comfort him and then attending the wedding. If reconciliation is not possible, then a promise to visit later should be made to avoid displeasing relatives.