Happy- and NOT going to Stanford will also not change your life trajectory in a meaningful way so I think the focus here on ONE grade and ONE college and ONE path (following the sister) is terribly misguided.
Kids with perfect academic records get rejected from Stanford all the time, and the message should NOT be “life’s not fair” or "boy you messed up sophomore year by doing/not doing ABC.) The message from the parent needs to be “wow, it’s a competitive pool of really talented kids and we’re so proud you did your best. And how cool is it that you get to pick from CMU and JHU and Wash U and Columbia, all of whom have really strong programs in stuff you’re interested in! We are so proud of you!”
To start- sophomore year- perseverating on admissions to ONE college and ONE math grade sets the stage for some really sad stuff down the road. And does the sister need to feel guilty that she got in and younger sibling didn’t?