In Louise Erdrich's poem, "A Love Medicine," I think there may be two stories being told. I believe there is the story of the speaker's sister, Theresa, and how she is abused, when she his hit in the face ("she steps against the fistwork of a man...") and then when he steps on her face ("...and his boot lants its grin / among the arches of her face...").
The speaker does not see the damage cruelty, but she sees its "aftermath," and it hurts her deeply. This is not just a sister, but someone—though not like the speaker—who Erdrich compares to a graceful and beautiful dragonfly.
As I reach the end of the poem, I notice that Erdrich uses the line, "I find her..." three times. This repetition might signify that the abuse Erdrich speaks of goes beyond Theresa. If this is so, it is easy to assume that the rising water is symbolic of an impending threat, and the storm that brings the rain may be symbolic of violence.
With these things in mind, I think Erdrich may be speaking to the audience literally and figuratively. Theresa may well have been brutalized, found by her sister. This experience may have opened the speaker's mind to the plight of other women in the same circumstances.
When Erdrich writes, "Sister, there is nothing I would not do," I believe she may in fact be speaking to Theresa, but she also may be advocating for the protection of all abused women, and their liberation from abusive situations: she may be standing up to say something must be done, and she will be there to help. "Sister" may be used here in a universal way, including not just Theresa, but the speaker's "sisterhood:" all women.