And, Sakura and Rin's situations are in no way comparible. Sakura
can't ask Rin for help, because Zouken wouldn't allow it (and would punish her severely for it), and further it might endanger Rin for her to do so. Further, Sakura has extremely low self-esteem, due to her abuse, and thus expects Rin not to care which, coupled with the fear of what Zouken might do to her if she tries and the fear for Rin's own life, explains why she dare not ask for help. Rin, on the other hand, has nothing whatsoever stopping her doing so, other than some vague promise to her long-dead father. She does not do it because she is more bothered about some stupid 400-year-old pact than about ensuring the well-being of her own little sister, and thus she
is to blame, at least a little.
I can forgive Rin for it at the stage of FSN, because, ultimately, she's only 16 and she's very much beholden to her father's idiocy, but if she fails to look beyond that in the future and attempt to reach out to her sister, then I would find it a
lot harder to forgive her for the suffering (and eventual death) that Sakura will be condemned to as a result.