Everything that has been shown so far between Uiha and Uraha has been just everyday sibling teasing. What people are calling Uraha's "blackmail" is a very mild form of that. In fact, they're on very good terms compared to many real-life sibling relationships these days. Uraha doesn't seem to go very far with her teasing, while Uiha is just extremely tsundere without resorting to violence all the time. I feel bad for Uiha at times, but she really has to get over her own short fuse, and soon.