How to get PangoFontFamily from PangoFont

["NAKAMURA Ken'ichi" <nakamura sbp fp a u-tokyo ac jp>]

Hello,

Now I am trying to add Japanese dual-width punctuation spacing 
to Pango FC shape engine.
(Japanese dual-width punctuation requires special treatment. See 
JIS X 4051)

pango_font_family_is_monospace() seems to classify dual-width font 
family as monospace. But, how do I get PangoFontFamily from 
PangoFont?
Or my plan is wrong?

NAKAMURA Ken'ichi