there
were two girls at the beginning and Jeanette was lost part way (perhaps she committed suicide from the abuse). Jeanette was probably in Therese's vision the stronger sibling and/or was protecting Therese, until her end. Therese need not have known that Jeanette suffered the same as she; or perhaps she did as both were accusing each other with their own version of the story.
In any event, after Jeanette is gone, to deal with the ordeal with her father, Therese recreated Jeanette within her mind. Therese's hopes that Jeanette's continued protection/support failed utterly, and thus Therese, from her initial trust and believe in Jeannete, began to turn into loathing Jeanette for her failure. Subsequent evolution of both sisters should be pretty much like already posted.
The Malkavian embrace strengthened and completed the division of minds, culminating in Jeanette being reborn in undeath with Therese "siring" her. I would think that if Therese had never loved Jeanette, she would never have embraced her, even if only mentally doing so. To embrace Jeanette simply as an excuse for weakness means that Therese has to admit she has failures and needs to have Jeanette around to blame, even if only in her subconsciousness. It would perhaps, be more plausible for Therese to accept the reverse; that Jeanette had embraced her instead as Jeanette would need Therese deal with Jeanette's weaknesses.
It is merely for the sake of argument that Jeanette really did exist. Kid SixXx has a really good post for a purely imaginary Jeanette given life by the Malkav curse.
Therese likely knows that she is Malkav but aspires as a Ventrue or wished that she was Ventrue and so acts out accordingly. Jeanette admits and wholeheartedly embrace being Malkavian.
Alternately, Therese would truly believe she is Ventrue, until it was pointed out to her. It could be reasoned that the Malkav curse caused Therese to believe that her sire was Ventrue, the Ventrue flaw can easily be enacted out by her believe alone, and there is nothing in the rules that say that a Malkavian cannot take on a Ventrue flaw as part of the Malkavian curse. As I understand, Malkvians, by rules, get to choose what dementia they suffer (as part of the Malkav flaw) anyway, and creative players can reasonably make
"believe I am Ventrue and suffer all their flaws as if it were real" as a dementia on character creation.
Similarly, not all Malkavians need to be utterly loony and totally incoherent. They are incurably insane, but insanity need not mean that they cannot be genteel and upperclassy. Not to mention that madness is also, story-wise, subject to the mental state of the individual at the time of embrace, to deteriorate, but also at an individual pace.
Furthermore, it would not be at all difficult for a powerful Malkavian to use any form of mind powers to impart into a young sireling's psyche that they "are of Ventrue, not Malkav bloodline" (see the rules reason above), if it serves the purpose. Or just for fun. Or as perhaps it was Grout himself that sired, as part of this research into the understanding and comprehension of the Malkav madness.
Edit:
On the Ocean House being related to the sisters, it is really interesting. Perhaps there is more to Therese's interest in the house, and perhaps Jeanette's as well beyond her tipsy facade. Or if could be that it's easier to reuse small decor than creating new ones; especially where time and money is short in coming.
