Kaa-chan no Tomodachi ni Shikotteru Tokoro Mirareta.

You're Naoki, and you overheard your mom having an embarrassing conversation about you with your next door neighbor Shiori. Yet even so, you're still looking forward to the next five days. You're close with your mom, but since you live together you sometimes wants to stretch your wings and fly a little crazy. You wants to be able to fap, without being concerned about your mom or anything!One time you end up attending an event for your mom's work, but the whole time you were thinking about that super hot porn video your friend showed you. And Shiori's there. You've been neighbors for 10 years, and it wouldn't be strange to call her your second mom. Well, except for the fact that you found her body incredibly sexy... You wouldn't mind losing your virginity to her.But just when you'd decided that was impossible, she starts to comment on how handsome you've gotten...[From DLsite]