Is it obligatory to fulfill a vow not to marry after the death of one's husband, especially after swearing an oath to that effect? And will a woman be with her last husband in the Hereafter?

A woman may accept a suitor who proposes to her if he is a man of religion and good character. Her promises to her husband to not marry after him are not binding, and her oath to refrain from marriage does not make breaking it forbidden; rather, she is obligated to offer an expiation for breaking an oath.