I think that when you give the egg to someone, that victim will be taken to an alternate universe where they are the ones that are holding the egg. In that universe, resides their doppelganger, waiting for them to fail so that they can replace the victims and act as their mental torture. The victims will continue to branch off the curse to as many people they can according to their level of desperateness and will, but I don't think that the victims in this alternate universe are real people, they are just there to fill in the original world's social roles and as mental torture for the egg holder. If the egg holder fails to pass it on, they will truly die and be replaced by a doppelganger in the real world where the doppelganger will pass on the egg to another victim, hence the last scene where mc is smiling sinisterly at her brother.