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.
Since you and your brothers share the same parents, grandparents and such, both of you descended from the same ancestor. And since the Mine's evil twin was a hideous mutant, it's safe to say that you both descended from the lineage of the good twins.
The whole Tomato Surprise makes sense in a twisted way; Yes, you kill all those people and things along the way. And think about the final "battle" — the whole incident that started the curse was an act of overreaction and gross stupidity. Witch curses family because ancestor cut off her hand. Ancestor cut off her hand for stealing a single freaking chicken. If you were the Good Twin, you're sure to realize just what a whopper of an Idiot Ball was in play and think of multiple ways that this whole mess could have been prevented; bribing the witch so she doesn't steal the chicken, stopping the ancestor from cutting off her hand (by persuasion or assault), offering to pay for the chicken that got stolen, etc. But since you're the Evil Twin, well, let's just not think too much about how stupid this whole thing is, take the path of least resistance, and make sure the old bag dies first.
Well, you have seen all the evil things that servants of beelzebub usually do (I.E. Evil in Egypt, Necromantic Nightmare, Mutant Mines and Knife Killers) So you can be pretty sure that the witch wasn't a nice person at all!
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 that the good ancestor you're controlling knew every single one of the zombies attacking him and was killing your friends, family, and loved ones over and over again with each kill.
You "defeat" the Necromancer 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 both angry and drunk, and suddenly a lot of people look sort of like you. How many people died that night to vigilante justice?
A man who feels that whores are replaceable objects even though he's fond of one of them.
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 before that curse could manifest.
All this crap happened over a stolen chicken. A single freaking chicken. A starving old hag takes your ancestor's chicken. He cuts off her hand. She escalates by cursing the bloodline past and present. So we have all those human sacrifices at the pyramids, at least six hookers killed in Whitechapel, however many "conscripts" the necromancer claimed, dozens of cultists and miners... and you top it by giving her a Cruel and Unusual Death where she loses her hand, gets hit in the face with hallucinogenic acid, gets shot through the eye, and her throat slashed. And all this over one damn chicken.
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.
