Yeah, I don't think that Buffy is Kendra or Faith's "mother" in the same way that Darla>Angelus>Dru>Spike are a family, because, well, she didn't make the choice to have offspring — until Chosen. I think that's why the "Sister Slayer" nomenclature works for me. Siblings don't have to choose or do anything for new ones to arrive, while parents need to be a bit more involved.

Now, there's always the argument that not all parenthood is planned, but there's accidents and then there's accidents if you know what I mean.