Thought of 2 variations for a scene.
Variation 1
Jess walks into the room in her underwear holding some knickers/thongs. "Hi brother, I'm going out tonight and I want your opinion on which knickers I should wear. I'll try some on for you and you tell me which is best. Don't get too excited, I want your hands where I can see them. Now turn around while I put the first pair on."
You pretend to turn your head but the camera shot stays on Jess as she drops her panties and puts on the first pair. "Ok, what do you think of these?" She shows you from the front and back. She models 2 more pairs for you but as she goes to pick up the last pair she says "Oh wait, I can hear my phone ringing in my bedroom, hold on, I'll just go answer it." she goes out the room.
You get up and walk over to the pair of panties she was just about to try on. The fake cock comes into camera shot and cums on the crotch of the panties. The cock goes out of view. After a moment Jess comes back in the room. "That was nothing important, Ok, were where we? Oh yeah, I haven't tried on this pair." She picks up the panties you have just come on and puts them on. "What the... these feel all damp" she says curious and she starts rubbing the knickers against her pussy to feel what it could be. "Whats all that?... have you done something to them?... You didn't come on them did you? Oh fuck, my brothers spunk is rubbed all over my pussy..." the scene ends.
Variation 2
Jess walks into the room in her underwear holding some knickers/thongs. "Hi brother, I'm going out tonight and I want your opinion on which knickers I should wear. I'll try some on for you and you tell me which is best. Don't get too excited, I want your hands where I can see them. Now turn around while I put the first pair on." she says with a cheeky smile.
You pretend to turn your head but the camera shot stays on Jess as she drops her panties and puts on the first pair. "Ok, what do you think of these?" She shows you from the front and back. She models 2 more pairs for you but as she picks up the last pair she says "These feel all funny... all stiff... have you been messing with these? Have you been wanking in them?" she smiles.
"I can see my the huge hard on you've got that you liked me modelling for you just then... I didn't know you like wanking to my panties... does it turn you on to have have you sister wearing panties that you've cum in - your dried cum against her pussy?" she asks smiling, she likes the idea too.
"I think we can go one better than that." she walks over to you, kneels down and wraps the panties around you cock and starts wanking you off. "Bet you never thought your sister would be doing this for you." she tells you to explode and drain your balls over her panties. She hold the panties in front of your cock and you cum on them.
"Wow, you cum loads. I think I'll wear these tonight." She slips the cum wet panties on. "These feel so good, mmmm my brothers hot cum all over my pussy is so hot. Well, I best be going, see you later." she smiles and walks out the room. The scene ends.