Wearing panties to join the sleepover

Frank just wanted to be one of the girls
Frank stood at the top of the stairs and listened. From below he could hear his sister and her friends enjoying their sleepover. He had been banished upstairs and told that under no circumstances was he to come downstairs. Downstairs was a girls-only area for the night. However, Frank was determined to join in the sleepover one way or another.
From the shrieks and screams of laughter they were having a good time. They were chatting about their favorite bras and which bra they wore under which top for the best effect. Frank wished he could join them, not as they thought, to ogle at them, but to join them in their fun as one of the girls. For Frank had been crossdressing for years and he couldn't bear to let the evening go by without joining in.
He waited in his room for about an hour until he could hear his sister Alison, or 'Alie' as he was known, and her friends were thoroughly engrossed in their party. Then he took off his clothes, adjusted his long hair and them quietly went to his sisters room. He headed straight for her panty drawer and pulled on a pair of blue stripped panties and some matching socks to keep his toes warm. Then he padded to the top of the stair and listened to the party below.
The girls were talking fashion, discussing low-rise jeans and just how tight they could be without being uncomfortable. The big question was whether or not it was cool to show your undies at the back of your jeans when you bent over, and if so what kind of undies were best? Briefs or thong?
Frank closed his eyes and imagined that he too was downstairs amongst the girls being girly. He believed that a thong was best if you were wearing low-rise jeans and wished he could tell the girls his view.
Then to his horror he heard the upstairs toilet flush. There should be anyone up here!
The toilet was between him and the bedrooms and there was no where for him to run to.
A moment later the door to the toilet opened and Jodie, his sisters best friend walked out. For a moment her face registered confusion and shock, and then it gave way to a big grin. She planted herself squarely in corridor blocking his retreat.
"ALIEEEEEEEEEEEEE," she shrieked....