This mother won't let him burn in hell, and that's her promise to her son. She finds out some things about him that weren't acceptable to her and some were downright sinful. She says what she is about to do helped save his brother's soul. Now it's time for her to save his soul. That's her duty as her mother. Hence, she offers herself to her son so she can be his personal cum-dump and lets him ejaculate in her as many times as he would like so none of his sperms go to waste.