Haven't played that far in game, but here's some feedback.
Improvements:
Fast skip text button.
Yui's art could be improved at home (button that opens interaction menu), looks inconsistent and different than dialogue art.
Larger UI buttons, Diary, Inventory, Characters, Save, Title
Add Load button, now you have to go back to title to save scum.
Make imouto made meals consume food items starting from the cheapest item.
Make cooking fail, less stamina recovery? mood changes etc.. easy to implement.
More home interactions.
Lurking outside toilet and shower scenes are missing?
Bathing scene needs some dialog and interaction options.
More bed minigame poses and actions.
Block penetration early in game, no build up at the moment.
Her waking up have no consequences.
Save menu, make save menu close after saving, now you have to press closed button.
Closed button should say "Close" instead of "Closed", could also be just "X"
Some backgrounds have different style than others. All backgrounds should be harmonized.
Battle:
Characters with only one skill, activate skill selection by clicking character icon.
Typos:
Many menu texts are in lower case, ex. bookstore -> Bookstore
Many dialog begins with lower case letter.