well, Riza's father taught Roy alchemy.
Just like a soap oprah, Riza's father croaks in the middle of some important speech he is giving to Roy. Riza walks in, Roy is about to cry (sissy, lol) Riza has a meltdown. Riza's father's last words were something along the lines of "she knows the secrets of my work. (uhm, no idea what that work was...) A later scene during the Ishbal rebellion where a rebel is about to kill Mustang when suddenly a shot drops the guy. We later learn it was Riza, who joined the military after Roy's apprenticeship ended.
Sorry, the details are a bit vague. haven't read those chapters in awhile.
Anyway, hope that was of some help.
(If u want to know more i've made a point of re-reading those chapters.) or u can just read them yourself if u'd like.