You can't make sister mod, only anonymous evil hackers not affiliated with you can.
Personally, even though I'm usually a fan of that, I couldn't care less about it in this game. Actually, I was kind of glad that it's just nice girl for a change, there was too many sisters in games already. But it's because I'm not primarily incest fan, I just like the forbidden / dangerous / "must not get caught" part of it, and there are not the right conditions for it here. True incest fans will have different view, but you need to be carefull. Give them sister and soon they'll demand mom, aunt, grandma, ... Hopefully it won't come to that. And you're already in dangerous waters with N-word stuff.
And if it helps you, thinking about game improvements sounds much better than smoking or using FB to me.
My view on incest is similar to yours. It's not the familial connection, it's the risk. Another aspect to it is the aspect of 'if she'll fuck her brother, surely she'd fuck me if I were there, right?', similar to how many people - primarily younger people, I believe - enjoy other fetishes like the use of food dildos or even to a more extreme degree, animal porn. If she'll have sex with that dog, surely she'd want sex with me too.
I know there are other reasons for them which apply to most people. Some people just like dog penises and if that's you, more power to you.
Side note, when I was 13, I thought pee was cool for the simple reason that it was something coming out of a vagina. I visited some really dodgy sites watching those videos.
On mods. If Patreon asks me to NTR stuff, I will. It wouldn't be too difficult to turn that into a mod. The problem with the sister mod was that it was incorporated into every block of code. I had created a variable named modSister. During dialogue, I checked to see if this was true or false, and the result dictated what dialogue was used. If true, 'you cum on your sister's face.' if false: 'you cum on Sarah's face'. I thought I could get away with this so long as I didn't give out the code to activate the variable on Patreon, but recent actions by the site have made me doubt this and believe it's not worth the risk. I took all the dialogue from the scenes out and I can create a mod with them, but it'll be an external mod and won't be integrated into the basic dialogue nor the interactions with Sarah and others, aside from the actual scenes.
The main drive for me wanting to have a sister mod is that, imo, Sarah fits the 'little sister' look so well, and that my favourite path - the pet path - also suits this. The original story and paths, which I've deviated from quite a lot due to limitations of time, also fit it very well as they were designed around it. The game's initial premise was that you and your sister had just escaped and now you are her authority figure. It's about how you use this authority and what you make her do/turn her into.
I'm rambling again. This is what happens when so few people talk when I release a version, the words get all bottled up. It doesn't help the person I speak to most doesn't speak English.
Sure, who would say "no" to that. But too often it doesn't end there. For some reason, there's this common assumption that if MC is after sister(s), he must be also after all female relatives. And worse, everyone seems to want them to be "MILF" types, but with significantly different meaning than the acronym. Instead of "Mom I'd Like to Fuck" they are mostly "Mom I'd not touch with ten foot pole and I'd run screaming if she tried to touch me", monstrosities like in Dreams of Desire and similar. I'm grateful for every game where something like that doesn't happen.
Seeing as you posted again. I completely agree. In a game like Big Brother, it was always a disappointment to me when I read a change-log to see the focus was the mother.
I feel many people are interested in the concept of incest, especially within the adult game community. In reality, the vast majority of incest cases are just two siblings experimenting, or an older family member being attracted to a younger one and using their power over them to make the relationship sexual. There are, I believe, very few people who just want to fuck everyone in their family and has actually tried. The reality is that siblings are often very close, share similar deep experiences and are naturally drawn together. I'm not speaking from experience, I don't even have a sister and I've never known someone in an incestuous relationship, even a casual one. I have researched the topic due to simple curiosity thought, and these were my findings. Note, my research was very American-focused. Elsewhere in the world may be very different, I don't know.