During the civil war in US there were states at border where friends and indeed brothers often faced each other across battle lines, and families were divided forever or at any rate for a while by their opposing positions on slavery and secession.
Also, the phenomena then unacknowledged but quite widespread and very well known of slaves being mothers of the master's children, whether through rape forced or maerely an assumed lack of any will on part of the slave - droit de seigneur - is now acknowledged somewhat, but has not quite been given the treatment in social justice it should be accorded. Recall the Jefferson clan furor when DNA tests proved the line of Jeffersons through the slave that was in fact his wife's half sister, who had been given to her at her marriage as her personal maid.
This book deals with both these phenomena, rather delicately, with the slave and the master in a more equitable relationship - that of a more caring, understanding, love, which could happen too, while it is not to deny that rape was a phenomena widespread. And also she is a half sister of the mistress in this story, from what I remember after more than three decades of having read it.
Not that that (blood relationship) is the reason that is either ascribed by the writer, or intended in any way by her to be understood by the reader, for the woman's being a person of many superior virtues in many ways.
It is only that such virtues in slaves were then acknowledged only privately - if at all - within the family generally and perhaps tacitly by the intimate visitors of those families, but not acknowledged much less ever discussed in public in any forum. Tf they had, there could have been no excuse for continuing with the horror much less going to war over it.