"Sisters appear to encourage more open communication and cohesion in families. However, brothers seemed to have the alternative effect. Emotional expression is fundamental to good psychological health and having sisters promotes this in families."
I think our culture consistently denigrates the positive impact that women have on men and on other women, or we are only comfortable talking about women in sexual terms. But actually women are valuable as sisters and as friends, in my opinion. People talk about the brotherhood of man, but the support that women offer as sisters, and in sisterly ways to men and women who aren't strictly family members, is an overlooked heroism.
The research of course can only look at the quantifiable - how many actual sisters one has - and not at the unquantifiable sisterliness one might receive from women whether 'sisters' in fact or not.