How big is this “small” cage? She needs to be in a cage that is a minimum of 18”x18”x18” although one 30”x18”x18” would be better. Move her to a different room and give her out of cage time separately from the others. You can try letting them out together in about 3 weeks under close supervision to see how they do. However m, be prepared that she may have to be housed separately on a permanent basis if the aggression persists.
What is the length and width of the cage? Does it meet the specifications I gave you previously?
Going by the picture looks much too small for one budgie -- more like a cage one would use for transportation to/from the vet. I can't find the specifications on the link you sent. By the way, Petco will accept the cage for a return assuming you have the receipt and everything that came with it. Clean up up before returning it.
There is no point in being upset and your birds pick up on your emotions. Why not ask for an “advance” of the funds and then agree (and follow through!) with doing additional chores to pay your parents back?