You must have high affection with both cousins (Yui and Rio) and have unlocked sex with your aunt. Trigger it by letting one sister catch you with the other while their Vigilance (alertness) is at 100.
Once the threesome is unlocked, maximize your aunt's vigilance while with the cousins to trigger the foursome event. summer memories my cucked childhood friends ano exclusive
Success depends on balancing "Homework" levels and "Affection" with various heroines to unlock story events and lewd scenes. Understanding the "Cucked" and "Exclusive" Context You must have high affection with both cousins