The joke is a pun. 'Still' in this case has two meanings. Still means to continue or remain doing something. A still in terms of alcohol bootlegging like in the prohibition days is a distilling apparatus for making alcohol.  Saying "I loved her still" when she's the daughter of a bootlegger could mean that he loved the still she used, not that he continues to love her.
She was only a Bootleggers daughter , but I loved her still .