His sister is a very good fencer, but Cassian has seen her duel thousands of times. He has not been witness to Jyn doing so in person, just the livestream from her last tournament. If his eyes are on Jyn, he is simply studying her technique and trying to decide whether or not to bet on his sister! They're putting on a show, everyone is watching.

But he cannot take his eyes off Jyn at all. The way her dress swirls around her legs, the smile on her face, the tight grip on her saber that flexes her muscles. He would be lying to deny she is utterly beautiful right now, and fortunately he can admit it strictly in his head, where no one else can hear it, especially his sister who he desperately hopes loses. There's a stirring in his chest, and he is trying to decide just how worth it it'll be to challenge Jyn himself. He wants to so badly. He knows this is more the both of them showing off than a tournament-esque battle, but he's so fascinated watching Jyn move.

If he was paying more attention to his sister, he might have noticed how purposeful Shara is in her moves, the way she turns the tide into something of a draw. How she isn't trying to win necessarily, just trying to push Jyn into being the star of the show.

His sister is a jerk, but he's too distracted to notice.