From: Rabbi Moshe Sofer
To: Unknown recipient
A woman who remarried is not obligated to observe mourning on the day of the gathering of her first husband's bones (likut atzamos), and possibly not even primary aveilus if death and remarriage preceded the formal mourning. The Chasam Sofer rules that the second marriage completely severs the halachic kinship (kirvah) with the first husband, supported by a kal vachomer from the laws of tumas kohanim and by the principle that displaying such mourning before the second husband violates the Torah's ways of pleasantness and peace.
Source: Shu"t Chasam Sofer YD §355