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The curse happens in sometime in Medieval Europe. So all the scenarios make sense. Family is spread out through Europe and America. However this doesn't explain the fact there is an Ancient Egypt level. Unless the curse can go back in time this doesn't make sense.
"For spilling my blood, I shall call forth an ancient curse to poison yours. Two thousand years ago one of your Egyptian ancestors had twin boys. One was good, but the other was cursed by an evil pharaoh and turned into a monster. I call upon the curse of the Pharaoh to return. Once more, in every generation in which your family bears twins, one shall belong to Beelzebub."
The plaque for the Egyptian waxwork also explains this.
You being the evil twin seems like a last second Ass Pull. Until you realize that's why you didn't hesitate at all to kill so many people and things on your journey.
Actually, the prologue explicitly states that your brother was the evil twin. When you awaken him at the end, he says that the witch muttered something before the man your spirit was within (who wasn't an ancestor, as your ancestor was the one who chopped her hand off) turns into a demon. She was unable to curse your bloodline but managed to curse only the man you were inhibiting, leaving a possible Sequel Hook that never came to be.
The PC Adventure-RPG Waxworks seems to run on Fridge Horror. At first glace it's your typical "horror game" with evil plants and zombies. When you think about it, however, it gets worse. Spoilers Ahoy!
The graveyard area has you hunting down an Evil Twin necromancer in a zombie-infested graveyard.
Towns weren't too big back then. Chances are you know every single one of the zombies attacking you and are killing your friends, family, and loved ones over and over again with each kill.
You "defeat" the Evil Twin by summoning your ancestors' spirits to somehow turn him into a baby. Totally weird but okay. And then you realize, he's now a helpless baby in the middle of a zombie-infested graveyard. And since your goal is to kill the evil twins...
A Full-blown lynch mob roaming London looking for anyone who looks sort of like you. Add in the fact that Gin was very popular in that time period increasing the chance of the crowd being angry and drunk, and suddenly a lot of people look sort of like you. I wonder how many people died that night?
A man who feels that whores are replaceable objects even though he's fond of one of them.
Jack falls into the river and disappears. It never said that he died.
Throughout the level you are fighting various pods, vines, and "eyes". All of these are attached to a single entity: your brother. Every move you make to defend yourself is blinding him while causing excruciating agony.
In the final battle you win, only to become the evil twin. Your brother now will be forced to kill you as the cycle repeats itself.
Actually it's only revealed that you were supposed to be the evil twin the whole time, but you killing the witch stops the cycle.