Thanks to his sister's ego

He say his sister is that kind of person that likes to show everybody that her way, is the right way and that she can do everything better than everyone else. He wanted to know how far she would go so he started talking about how his ex-girlfriend was good giving handjobs. She simply coundn't accept the defeat without trying it, her ego simply couldn't let it go. He was surprised with her skills but if he wanted her to do it more times he needed to tell her that her ex-girlfriend did it much better and she needs to improve.