A story about my favorite childhood friend who calls me big brother and ends up having sex

A young man and his cherished childhood friend, who affectionately calls him "big brother," are suddenly transported to a fantastical otherworld. As they explore this unfamiliar realm and seek a way to return to their original world, they encounter a series of steamy and provocative situations. The childhood friend often finds herself at the heart of these risqué adventures, blending the thrilling elements of isekai fantasy with erotic undertones.