In both cases, they will already have reached the point where they are sexually active with you. The straddling scene with Silvie can only be triggered after her first time and the same applies here since, by the time you get the sister to enough interest and trust to do it, you will have already had sex multiple times.I'd love to know if you can trigger the scene if you have nothing done with her yet.
Like, really nothing, no stealth nut inside her when she is asleep and no cumming on her back.
Similar to Silvie when she gets desperate and makes the first move by straddling you in your sleep.