Figured it out. The little sister is like all women, just keep giving her stuff. For example, hand held games. I'm not 100 percent sure how I triggered the first sex scene with her (bathroom footjob), but I began by showing her the dildo, which made her scream. I then kept giving her a steady...
I cannot, for the life of me, figure out how to get the little sister for sex. I know, with the precious 2 interactions you have with her at a time, you have to do some sort of combination of items to get her going, I think. I've already tried showing her a vibrator/dildo, but all that got me...