Sander on a summer vacation

This is the story of a 19-year-old boy who had to accompany his aunt and cousin on a vacation to their summer home. The boy in question is named Sander. He had hoped to be home alone while his folks went on a cruise. Sander is a little short for his age; he is 5'2", has dark hair, and blue-green eyes. He had an extremely feminine build and face. Many people initially believed he was a girl. Heidi, his mother's sister and Hanna his cousin. This is how it began.