It is a good try for your first game.
Some quick observations:
It is unnatural to have the head facing squarely at the person they are talking to all the time. I do not know for sure, but I think A head shouldnt be facing squarely at someone less than 50% during a conversation. Think of it like a method of exclamation or significance... When you really want to drive the point home, or show your dedication to the things being said, you put the face square on the MC.
Eyes blink, close, squint, wink, and the eyebrows have a mind of their own during a conversation. The eyebrows will often be an indicator what is really being meant despite what is being said. Smiles, frowns, purses of lips, biting lips... There are so many more ways to show emotions without word with just the face.
I only mention this because the conversations were downright creepy, with unmoving, unexpressive faces, that stared you square in the face unblinking for every conversation.. You have the blushes down, just take it a few steps forward.
Lastly, the writing need to be written out story mode, and edited by an editor. Then rewritten to follow a VN, more than one draft to make sure it is smooth and makes sense. All those little things that are narrated needs to be conveyed somehow either visually or through inner dialogue. You have managed so far on your memory, which is amazing, but it doesnt make for a great consistent story.
More with the story: The premise is cliche... not in general, but on this site. You need to develop the story more, introduce some external plot devices, like the job promotion, but more of them. What do these characters care about, what is happening in their lives that is interesting besides the fact that they are developing an incestuous relationship. The problems in history class, and with megan are good starts, develop them into real plots. Try rewriting the story from several perspectives focusing on the outside influences, see what you can figure out that is interesting.
Develop the characters, one tool or method you could easily use is remembering. The MC has been away for 4 years, have mom and sis sit around reminiscing about past events. Give the characters some depth, show some flaws and strengths.
I am not an expert in this, just an outside observation.
Thank you for sharing your hard work with us. I look forward to future release.