Sam felt uncomfortable wearing his sister's bikini. These feeling grew when he noticed some tingling in his chest and in his crotch. He got scared and wanted to get out of the bikini.
He reached around his back to open the clasps of the bikini top. With great difficultie he could reach them. Trying to open the top he lokked down at his chest just in time to see to mounds of soft female flesh forming and filling the cups of the bikini top. The mounds were topped with erect nipples poking through the fabric of the top.
Sam was still uanble to move a finger while looking at the tits he now had when the tingling between his legs got more intense. Fearful he watched the bulge in the bikini bottom shrinking until the whole place was flat.
He wanted to pull down the bikini bottom to see what happened in his crotch when he felt something touching his shoulders and back. Looking up he noticed long red hair coming down from his head and reaching down to his butt. His face also head taken on a female exopression as did the rest of his body.
When the tingling finally stopped Sam saw a teenage girl the age of his sister staring at him out of the mirror.