Blue-
Some offenders are 'situational offenders', who do not necessarily PREFER a child as a sexual partner, and would perhaps prefer an age appropriate partner, but instead accept a child as a sexual partner because the child is available, and situational stressors (financial/relational/psychological/physical) exist.
It is possible this describes your situation.
That said, most child molesters have an 'age and sex of preference', and choose children to molest who fit their preference.
Perhaps you were his preference, and your sister was not.
This information comes from expert medical studies..
Hope this helps--
Kimberlee D. Norris