Recognizable, effective, successful.
Actually this is not really the band's logo but their own record
company's Merciful Release logo!
It also appears on the early singles labels of other bands
that had signed to M.R. like the March Violets, Salvation and
James Ray and the Performance!
But it got associated only to the Sisters, I think mainly due
to the success of the band and its consistent use
in every single release, t-shirts etc.
It's roots go back to 1858 and Henry Gray's anatomical studies.
Here's the proof...