Again, context of the situation matters. I suspect that Katie will initiate the play during the movie, and Katie and Jenna will be having a tease-worthy whispered conversation while she is stroking MC under the covers. At some point, Katie will ask Jenna about her confidence, and Jenna will reach in to show up her sister. At that point, it's just sibling rivalry to get MC off, but would be an important first step towards acceptance of sharing MC.
If there is a pairing with Katie and Jenna planned, then the described situation is what I‘m thinking would be happening.
However, I strongly doubt that Katie would want to share with her sister, after what she says at the beach house. Maybe they justify the sharing by having Katie see Jenna as inferior to her, such that she feels there is no danger of Jenna stealing the MC away. To avoid incest, they would have to come up with something like „just don‘t do it when I‘m in the same room.“ this would come from Katie as I would think Jenna would love it if someone is watching her and the Mc.
There are a lot of scenarios that could play out here though. My guess is, at the start you get the choice on who to sit next to, which will earn you points with the chosen partner and jealousy if you are dating someone and not choosing them.
Next, you basically get some lewd content with the chosen partner during the movie. Now, this could be it for that scene already. Although things might get spiced up, as Monica knows if you are interested in one of her daughters and the Sisters each might get suspicous during the movie.
So ultimately either nothing happens and you get a lewd scene with one of the three, possibly after movie sex a la „Mc and me are staying up a little longer, you can go to bed already.“ or this will create a meltdown where it‘s discovered that MC is fooling around with one or more of them. I doubt the latter because both Jenna and Katie want to tell their family that they are dating the MC, but the MC convinced them to wait with the announcement. So I assume nothing major will happen just yet. It will only add thrill by being almost discovered.
