Hey man, I like youre game so far, there are a very few sexual horror VNs out there, so it's nice to see people still try to think out of the box.
There are a couple of things I have noticed, for example, the house in the middle of the woods, the exterior is in fact in a bad condition, but the interior is immaculate man, the walls, the appliances, the lighting, all of those devices are modern and in a flawless condition. What what exactly does the MC needs to do there, it seems that house exterior walls just need to be painted and the garden needs to be worked on, but it's not like there is anything urgent and critical by the looks.
I think the players also need to know a bit more about the characters, right now, we don't know nothing about their background, are the MC's sisters study, work, or both? What does the MC do, besides being asked to do some "work" on the house? It's one thing that we got a horror setting but unless you are trying to make the story running on a short period such as a week or less, then the characters need a life to be humans so then the players can develop some engagement to them and their story. In other words, they don't have a purpose, the two sisters are just hanging out, the MC is working on the house, and then the mother is... working somewhere... I suppose. So my point is, monsters or aliens aside, they to live, they need to have a purpose in life like real people, study, work, run a business, love somebody, get married, raise children and have plans for the future.
I never liked those stories where the world revolves around the MC, every women wants the MC, and MC gets incest activity without much input. This does not apply to your game so far, since you have only done an introduction so far and it's too early to say anything, but I just wanted to share my personal point of view.
By the way, the models look nice, I like them all so far.