Two sisters draw a conclusion

The sister noticed someone had been wearing their unerwear

Mandy came storming into her sister Rachelle's room waving the frilly red thong panties in front of her.
"Have you been wearing my panties?" she demanded. The red panties were among her favorites and she wanted to wear them that evening for a date that she had arranged.
"No, of course not, why would I want to wear your undies when I have my own," Rachelle retorted. "But I'm glad you brought the subject up because I was going to ask you the same thing."
Rachelle was sitting cross-legged on her bed with all of her panties laid out around her.
"I'm missing some of my undies," she explained. "I thought you had been borrowing them."

"No, I haven't. But these red panties were in the laundry and I didn't wear them recently."
"Look, I'm missing my apple green thong from Victoria's Secret and I can't find any my boy-short panties. You could at least ask before you wear them."
"Rachelle, I have not been taking your panties!"
"Well I haven't been wearing yours either."
The two sisters looked at the panties piled on the bed. They had both noticed over the past few months that not everything that they put into the laundry always came back on time. They looked at each other and the same thought went through their minds: David, their brother.
The two girls frowned at each other for a few moments without saying anything. It was Mandy who started to smirk first and then Rachelle cracked into a laugh too.
"You don't really think David has been wearing out panties do you?" Mandy giggled.
"I don't know, but I bet he wants to."
They both laughed at this.
"Let's ask him," Rachelle said.
"Just like that? You can't!"
"I can and I will," Rachelle said in a determined way.
The girls looked at each other again and then both shouted out.
"DAVID!
In his room David looked round. There was something about the tone of his sisters' voices that he didn't like.
"What?" he replied.
"Come here," came the reply.
David went to Rachelle's room. As he walked in he saw Mandy, Rachelle, the panties spread out on the bed and he knew he was in trouble. Just stay calm he told himself but the knowledge that he was wearing Rachelle's pink boyshort panties right then only made him panic even more.
"Have you been wearing our panties?" Rachelle demanded.
David felt his face flush bright red and that was all the answer the girls needed...