Use of "respectively"

It's not needed because, as you said, nothing is being enumerated. If, instead, it said "he has a sister and a brother who live in southern and northern CA, respectively", that tells you something -- the sister lives in southern CA and the brother lives in northern CA.

I don't know if the usage you found is wrong or just unnecessary.

It doesn't make much sense because I'd expect it to be like this (for example):

He has two sisters, one younger and one older, who live in southern and northern California, respectively.

In this case you could associate the "couples": younger - southern, older - northern.

The usage you reported is unnecessary and confusing, I don't think it's a correct usage, since respectively refers to something previously stated which has been enumerated.