Personally, I don't mind Kim and liked her going behind the MC's back with his mom as a plot device. I'm most definitely not someone into NTR, that will kill a game for me instantly, but I also don't read this as NTR and I think the conflict between friends could be a fun/interesting plot device. Especially since now we know they are going to be going back and forth into the past to "fix" things butter fly effect style over time, who knows what else will be changed, including their personalities and tastes. As for it being a dick move on her part, well yea, but that's a great conflict point to develop, and honestly who among us is that good of a person? you have a once in a life time opportunity to get with someone you've always been attracted to, friends sister/mom/future girlfriend/ex whatever. Depending on your relationship and your moral compass many will resist, but many will not as well. Repairing that and coming back from it is an interesting problem/story. Having everyone be a perfect white fucking knight all the time (a hilarious concept in an incest game btw) is boring. We need more new interesting stories or this whole fucking genre is dead.
To the author, i've followed you for a while,haven't always agreed with your choices(for example switching the scene between MC's mum and kim to just a kiss was.... much less compelling, making the little sister in the artifact go from curious and innocent to super slut in like, a day, also took me out of the story) but i'm still here. keep it up and keep writing what you like. When you're invested in the story it shows through, when you try to please everyone it turns into a dumpster fire of disappointment and despair. Cheers.