ДНК S06E27 - Выпуск 744. Выжила после оборта?

Two sisters found out that their mother buried the third one right after giving birth. One day they received a strange letter in a social network. Now they are wondering: did their sister appear in their life after 31 years and for whom, in that case, did mom wear mourning? Relatives of the sisters are absolutely sure that the same girl did not have time to be born. But why, then, did the adoptive mother of their supposed sister decide that they were related by blood, and is this so?