  It was shown recently that the heart of a twin cotorsion pair ((S,T),(U,V))
on an extriangulated category is semi-abelian. We provide a sufficient
condition for the heart to be integral and another for the heart to be
quasi-abelian. This unifies and improves the corresponding results for exact
and triangulated categories. Furthermore, if T=U, then we show that the
Gabriel-Zisman localisation of the heart at the class of its regular morphisms
is equivalent to the heart of the single twin cotorsion pair (S,T). This
generalises and improves the known result for triangulated categories, thereby
providing new insights in the exact setting.
