Look at human ancestry:
http://i50.tinypic.com/xm1tuw.jpg
The very first divide within Homo Sapiens itself is Africans vs
non-Africans.
Which is consistent with the out-of-Africa hypothesis which provides a
mechanism for all this btw.
If your deranged sister (or daughter) itches to practise exogamy and
marries a black, hey, good (bad!) for her; one wonders why she doesn't
take things one step further and marry a chimpanzee?
Afterall, no pun intended, it's all relative.