Cassian knows his sister set this trap but the unfortunate part is that she is his sister and thus knows exactly how to get him. He walked right into this, almost knowingly, but at the moment he can't bring himself to care. He'll just ignore Shara's texts later when he leaves without saying goodbye.

But he thinks about Jyn whirling across the floor, of her delightful defeat of the gold git and he wants to chase the thrill of her competition for himself.

He's not surprised when she immediately plows forward, that's kind of her style, so he swoops his sword up to block, the hem of her dress fluttering against his leg before it swings back around hers. He dislodges the strike and aims one of his own; it's barely five seconds into it, and he's already enjoying himself probably a little too much.