I agree. The thing is, when they're young, all their instincts are telling them to fly. So if all their instincts are telling them to fly, and they see the other bird can fly, but they can't? You're going to end up with a bird that has some sort of mental issue (whether it be low confidence, depression, anxiety, lack of motivation, or being a perch potato to an unhealthy extent). I agree with Donna. Clip none or both.I am not against very conservative trimming of the primary flight feathers. I hesitate to even call what I'm talking about "clipping" which implies rendering the poor bird unable to fly which is cruel. A conservative trimming allows the bird to fly but not too far and not too high for their own safety and to make him easier to handle and I do NOT want my birds flying high enough to land on top of my ceiling fan. But I would think that only trimming one of your conures would be kind of unfair to baby. Personally, I would lightly trim both birds or neither of them. Your sun needs to learn to get along just as much as baby does. Just my opinion and I'm sure others will disagree. I dont think there is any wring or right answer except I don't know of anyone on this forum who would recommend clipping a parrot's wings enough to prevent the bird from flying except when absolutely necessary.
I do think this issue could be easily solved, though. Let them out at different times, or in different rooms. As Donna said, put something over the top of the cage so your sun can't bite the baby. If the baby is too flighty, work with them in a bathroom or another small confined space where you can grab them in case of an emergency. You can even work with them in the shower (just physically in the shower, not with the water running) if your bathroom is too large and you're scared you won't be able to catch the baby. You should never catch them unless you absolutely have to, though.