I'm afraid you're wrong here. The threesome scene will have something to do with the panty party, just like the sex scenes of the other LIs. Haven't you noticed that there hasn't been a preview of a LI from the panty party yet? The reason is simple.
Sage's parents are getting divorced and Sage won't be up for a panty party. That's understandable. Jill isn't the type for a panty party and Isabella can't hide behind a mask like at Halloween. Josy and Maya can't attend the DIKs' panty party either because their brothers are DIKs and they certainly don't want them to see their half-naked sisters all evening. All good reasons why there won't be any LIs at the panty party.
This reminds me of something Jill once said. On her date with Rusty, the DIKs came and she became invisible to Rusty. In EP2, the MC said to himself that if he goes to Maya, he'll regret every minute he spent at the DIK party. To be honest, I would also regret every minute of the panty party if my girlfriend, or in Josy's and Maya's case girlfriends, were waiting for me like that.
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Sorry Dalli, but I gotta disagree with you, I honestly doubt none of the main girl will not be in the Party. (except Bella)
Firstly because it's a HUGE event, it will feature over 1200+ renders, so not seeing any of them for that long would be extremely weird.
Second, the reasons you gave for the girls to miss the party are quite weak.
J/M wouldn't give a fuck about their brothers (but I doubt they are using the BDSM wear in the party, that's just for the lewd scene). Like people said, they even played Beerpong in EP 10.
Sage wouldn't miss it as well considering she will probably want a distraction from everything that happened with her family, plus again the event if too big to be path specific to the Others path.
Jill already said in EP 10 for the MC not assume that she wouldn't be up to the Panty Party, even if she got a bit flustered when she heard about it. She could easily wear something a bit less revealing and no one would give a fuck.
The J/M scene is most likely either another Dare or a Christmas present that will happen outside the party.
I could just assume a threesome between Maya and Josy was a Christmas present for MC.
Josy writes that they have a present for him, he comes in and gets this scene.
This scene is more about fetishes than logic. There's not a single reason why Maya, who was devout and raised in a family with a strict religious father, suddenly became an anal slut.
It's just AVN logic.
Yep, makes sense, when that moment happens in game (MC getting a text about coming over to Maya's room) my heart will probably skip a few beats lol
That scene has everything to be my favorite, even if it was over spoiled.
Same location doesn't necessarily mean same branch. And remember that not every Bella path is reached by "cheating" on Jill, too.
There are plently of variations with Jill/Bella, I'm sure that if you hurt her, the invite to the winter retreat will have some special reason...
As for Bella/Jill we will see, events should be different.
They should, specially if you are in their faithful routes.
But there could be some minor differences if you dated both for a while...
The previews didn't shown much, so there were fine.
that Zoey running animation was damn good animated tho.. just saying
That's the first time I seen someone liking that scene lol
If the throuple are going to cower and hide just because Tommy and Derek are in the same room, they might as well never go to any party ever. That's stupid. Tommy can bitch and moan all he wants, cry "sisterfucker" at the top of his lungs while Derek just gives MC a dirty look, but the secret is out. Let them have fun out in the open, like when Maya gives MC a nice fat kiss in class, right next to Derek, who is not on mushroom atm.
Indeed, J/M will be at the party.
DPC probably didn't share any previews with them from there since he probably doesn't want to spoil their outfits, just like he did in the Hallowen party.
Which makes a lot of sense, I want those to be a surprise, he already spoiled too much already.
Wait... MC is going to have sex with both Jill and Bella on a Jill or Bella path? How do you even know that?
He didn't say that, only that both their scenes will happen in the winter event.
Yeah, and Josy is DP slut, because she need to fill both of her hole, and likely her mouth too , that is why she will get a Ballgag 
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Some Josy DP would be absolutely legendary, hope you are right!
Josy and Maya's outfits are quite a surprise but I don't think the "anal sex" scene that will probably happen is a surprise. In the Halloween episode, Maya said that she liked "everything with her ass". MC's helping Maya in episode 10 may have led both Maya and Josy to give her an award for "anal sex".
It's like people didn't saw their EP 9 scene, that was a huge tease for this scene.
I think you're misunderstanding what DPC likely meant by spoilers. Sex scenes aren't spoilers to him, but plot details are.
Some sex scenes shouldn't be spoilered though, specially ones which do somethign different like the EP 11 threesome.

Because MC licked and teased their asshole, and they enjoyed it, and decide to want to try. Because they never done this before but want to try new things.
You really start to talk nonsense now. it's like why Jill give Boob job and Blowjob now, and get her ass slapped ? They are just fetish why she doing it in her second sex scene ?!
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Speaking about Jill, I cannot wait to see her "progress" in EP 11 lol