Sibling Rivals

Warning: this story is for over-18s only and contain elements which some people may find offensive. If you aren't interested in golden showers, or aren't remotely curious, then please don't download or read this book.
Gavin is a new kid on the block - he meets Sophie whilst walking in the country, and they form an instant friendship. In spite of the sexual thrills she is able to give him, he becomes concerned about her dominant relationship with her sister, Val. And then, while Sophie's away on a foreign trip, Val appeals to his better nature and tempts him into her shower... and her bedroom. Is it revenge after the years of being her elder sister's subordinate?
What will happen when Sophie gets back? Will there be sparks?