A rare form of serial killer are Black Widows - women who kill their spouses and other family members - usually for financial gain.  Even more rare are Black Widow Sisters.  This week I'll share two stories of sisters who set up murder syndicates for profit.   #BlackWidowsofLiverpool #BlackWidowsofElSalvador #serialkillers  Photo: Margaret Higgins and Catherine Flanagan - The Black Widows of Liverpool www.patreon.com/onceuponacrime