Have they ever explained why he has an Asian sister?

...what? You think Liz is Asian?

http://www.imdb.com/name/nm3138882/?ref_=tt_cl_t3

..........WHAT?

A better question is why would they cast a girl 3 years younger than him to be his older sister.

Either way, I like her and the two of them feel familiar enough to be related.

She doesn't look even vaguely Asian.