I hear 'You call your mum a tiger' becuase it was written by an Australian, Mike Chapman. I'm sure I remember hearing an interview in the 1970s where Suzi said the lyric was "And we know that you ain't lyin'", which makes more sense to me. The animal imagery is so obscure I didn't know whether The boyfriend was getting it on with the mother, the sister, or both! Suzi herself never tried to understand Chapman/Chinn's songs - she just knew if they were hits. Obvious chord progression inspiration from Gary Glitter's Rock And Roll Parts 1 and 2 - cheeky!