In the books, it is established clearly that Hannibal was forced to eat his sister's remains, after she was killed by mercenaries and he was kept captive by said mercenaries. In the films, this is also brought up and said explicitly. It's a group of men that kills his sister and who give her remains for him to eat, only for him to realize it's her later on in his life.

 

Meanwhile, the TV show strays from the canonical backstory by leaving it vague and yet different. In season three, we're introduced to Chiyoh, who claims that a man she has been keeping captive for years killed and ate Mischa Lecter. From the start, it's already different— it's only one man guilty of Mischa's murder, not a group of them. Of course, Will has his doubts about that man's guilt, most notably about if he really did eat Mischa.

 

An episode or so later, Chiyoh confronts Hannibal about his sister's death and the fact someone cannibalized her. Hannibal says it clearly that "he did eat her, but he did not kill her". So, if he is to be believed, that man killed Mischa and then he ate her. If it was out of grief, to honor her (like with Garret Jacob Hobbs did to his victims), that is rather unclear.

 

The most stark difference is the fact there is no allusion to Hannibal being *forced* to eat his sister. For all intents and purposes, it is shown as if he did so willingly. 

 

But why change this? Why change this backstory detail that is only brought up in two or three episodes of the last season of the show?

 

My belief is that it is to avoid the trope of the Freudian excuse. The Freudian excuse is popular with villains and other unlikeable characters, especially morally gray ones. It is used to explain away all their misdeeds with a messed up childhood, abusive parents, bullying, and the like. It is named, of course, after Sigmund Freud, and more things about it can be found in TVTropes.

 

I believe that the showrunners of Hannibal were avoiding the trope, to not make people feel some sort of sympathy in regards to Hannibal Lecter. While I can't speak on Thomas Harris' intentions with that backstory (as, again, I have not read the books and I do not plan to), it can certainly be read as some sort of Freudian excuse, of *oh naturally he eats people, he got messed up by the knowledge that he ate his sister.* 

 

But here it is deliberately avoided, to say that Hannibal was messed up from day one. 

 

Another point that is worth bringing up about Mischa in the show, though, is how him eating her out of grief does not make sense. Throughout Will's dissection of the Ripper's modus operandi, he points out various times how he sees his victims as pigs and that he sees something fundamentally wrong with them (being *rude,* as the case might be), that he takes the organs he thinks they don't deserve. He doesn't eat the people he doesn't find despicable— he didn't get any of the body parts of Abigail after killing her; he never killed Miriam Lass and never ate the arm he amputated. He only kills people he finds pests, and he compared Abigail Hobbs to his sister, saying that he reminded her of him.

 

Of course, we can pin down the fact he never ate Abigail to the fact he had to flee. We could also pin the fact that he ate Mischa out of grief on him being young and still not having his strict blue-and-orange moral code. But I find the differences between the canon background of Hannibal Lecter in his different appearances in media very fascinating.