I'm working off of memory, but you need to have both the drug and confession for the good ending as well as a high enough intelligence and willpower stat to mentally link the two pieces of evidence together and to help your stepsister come to a realization. As long as you pass those two checks, the next and final step you need to do is just make the correct dialogue choices to get the good ending.
EDIT: Reloaded a save I had (hooray for compulsive saving!) and you have to have high Intelligence, Strength, and Dominance/Evil stat to have a breakthrough with your stepsis, provided you have the drug and confession. Fail any one of those stat checks and it results in the 'bad' ending.