It's unknown if Blanche plotted revenge straight from the "you bet I won't forget moment" up until she tried to run over her sister. One has to wonder what her attitude towards Jane was afterwards. Did she withhold the truth just to torture Jane even further (forcing her to act as her caretaker)? Did she keep the lie out of fear of what Jane might do for revenge if she found out? Or was she genuinely sorry and just didn't know how to come out with it?
Note that the book leans more towards the former interpretation, revealing that Blanche deliberately kept Jane from seeking psychiatric help because she was afraid that, as part of the therapy, Jane might remember that she was the miraculously escaped victim of Blanche's hit-and-run attack, rather than the assailant who crippled Blanche like she believes herself to be.
Jane with regards to how much she cares for Blanche. In the opening, although Jane is bratty, she still wants Blanche to be included (insisting her father get some ice cream for her too). Blanche describes Jane fondly in her youth, implying the sisters were close once upon a time. The fact that Jane is still caring for her decades later implies that there could be lingering affection that's getting squashed by her own bitterness and jealousy. When Elvira is killed and Edwin discovers the secret, Jane's first instinct is to run to Blanche like a frightened child. And when she discovers the truth, her first thought is not anger but sadness.
The remake implies the latter, with the sisters being cordial and even affectionate at times - before Jane goes off the rails.
Cult Classic: It was hugely controversial (it even received an X rating in the UK!) so of course it became a midnight movie - which it still is today.
Dancing Bear: A large amount of the film's publicity stemmed from the heated rivalry between the two leads.
"Funny Aneurysm" Moment: Mrs Bates says "that Jane Hudson makes me so mad I could kill her", and her daughter jokes "what'll we use?" - said daughter is played by BD Merrill. She would later publish a scathing Mommie Dearest style book about her mother.
Harsher in Hindsight: Jane Hudson is known as a drunken mess on set when she tries to be an actress, while Blanche is a perfect lady. The reverse happened to the two actresses that played them; Bette Davis was known for being a consummate professional who kept working for nearly twenty years after this film. Joan Crawford was known for being drunk and full of Narm, and would have retired from films before the decade was over.
Moral Event Horizon: Jane appears to have crossed this with her attempted murder and systematic abuse of Blanche. The twist ending reveals all this to be a subversion.
Jane's murder of Elvira would seem to remove all doubt, though it is debatable whether or not she is guilty by reason of insanity.
Narm: Lynn Redgrave's makeup in the remake. It's essentially the same as Bette Davis's— but the difference is that the remake is in colour. Davis's worked in black and white, and helped make Jane look creepy. Redgrave's, on the other hand, looks incredibly goofy. That's not to mention the Girlish Pigtails.
When Edwin's mother is telling him about the night Jane supposedly tried to kill Blanche, she describes the worst part—read: worse than trying to murder her own sister—as being found in a hotel room with a man she didn't know. Sort-of lampshaded by Edwin in his retort:
At the beginning of the film, they sell dolls based on Baby Jane and stress that they are "exact replicas." A doll modeled on a real life little girl would have been innocent enough at the time, but takes on far more sinister and creepy overtones today.
Jane. Poor, poor Jane. Wracked by guilt over something she never did.
Jane as a child qualifies too. It's strongly implied that their father pushed her into show business and allowed her to become the arrogant brat she was. He never disciplined her and made Blanche think she was The Unfavourite.
Blanche, over the course of the movie, culminating in her supposed death at the end.
There's more than an element of Jerkass Woobie in both cases, however. Blanche because she tried to kill Jane and then framed Jane for trying to kill her, and Jane because whether she was guilty or not, she still spent a significant amount of time psychologically tormenting and abusing her disabled sister.