根拠なんて、あんまり無い。
一応ちょこっとネットで調べたけどまあ両方にそういう説はあるらしいです。説は。
あと、続き物なんで、前の話は下のリンクからどうぞ

姉妹、対比＞ post #1653572

There's almost no basis to it.
(I tried googling about it in the web, but it seems that there are some theories about both of them. Well, theories.)
Also, this is a continuation. You can see the previous one from the link below.

Sisters, Comparison> post #1653572