You do realize that what you suggested in your first post is literally the polar opposite of baby steps, right? You suggested doing 12 workouts a week and getting her right onto an idealized diet plan, regardless of what she's currently eating.
Baby steps are already in place, and have nothing to do with what you suggested. She's working with a nutritionist, she's keeping a food log, she's eating more vegetables. What we're dealing with here is third or fourth hand info (some guys help a guy helping his sister).
This can be pretty common with the obese. You should take it as a glaring reminder that her issue is primarily psychological, like most disordered eating. If that isn't addressed (let alone "resolved"), the gastric bypass is going to be a band-aid solution at best.
Consider looking into overeater's anonymous, but don't directly approach your sis with this info. Maybe you could try calling them yourself and talk to someone for info.
It'd be better if you, or maybe your mom, walked with her to make it a team effort. However, start off aiming for just once a day, not "at least" once a day. No point in going from zero to 100% when going from zero to 10% will be just as effective.
Like BBB said, don't think of it as exercise. It's activity. There should be a distinct difference in how her gradually increasing her activity isn't similar to you increasing your own training.