In just four lines, the speaker describes living with a failure that has damaged a family relationship. The speaker's sister can't look at them without flinching, and the poem ties that reaction to a "little girl" who was never saved "from the torment." The speaker had made "pleading promises," so the guilt comes from a broken vow as much as from the harm itself. The poem doesn't say who the girl is or what the torment was, and that silence makes it feel heavier, as if the subject is too painful to name.
The speaker seems to feel shame and helplessness, and possibly a sense of being blamed. The title, "Your Fault," may be an accusation the sister makes, or one the speaker has taken on. Either way, the mood is quiet and bleak. Nothing is resolved, and the sister's flinch suggests the pain is still present every time they meet.