It also seems that Disney is also looking to cast a 9-year-old girl to play Cassian’s sister, though it's not clear how old she is in relation to him. It’s possible she didn’t appear in Rogue One because she died young, which could explain why Cassian decided to fight. In Rogue One, he did say that he “lost everything” and then “decided to do something about it," but tying that to the sister is pure speculation on my part.