Survivor’s guilt is certainly a plausible explanation for at least part Touko’s state of mind. The only trouble with leaning too heavily on it is that we don’t actually have any direct evidence that she even feels particularly guilty (it’s more than reasonable to think that she does, but it’s still just our inference). We do see what she tells Yuu in Chapter 10 along with her memory/thoughts at the end of that chapter, we see her dream/recollection of her sister’s death, and we see her being pensive in the cemetery. (The dream could suggest that she feels some degree of guilt, or it could just be a replay of her most traumatic memory.)
What she does directly say is that a) she couldn’t accept her sister’s death, b) she decided to become like her, and c) she liked the praise she got when she acted like her sister. That is, the loss of her sister opened up a role/niche in the family for her, and she took it, partly for herself and partly to honor her sister’s memory.
Even her discovery of what her sister was really like at school leads her to just say, in a very low-key way, “I have no idea what to do now,” as if a student council plan for some school event has hit an unforeseen snag rather than this new knowledge sparking an emotional/existential crisis.
You may well be right about guilt playing a major part in Touko’s ongoing emotional state; I’d be very surprised if it didn’t have some role. But my point is that, while we as readers almost can’t help reading these events in very dramatic terms, we’re basing that reading on inferences rather than on explicit evidence in the text.