It's related to Hero trait she owns. A chapter ago Red explained how traits affect personality and this applies to his sister (due to Hero's blessing, she tends to help people, for example)
But there is a whole arc based on this reunion and how to minimize the damage of Hero's trait (because it greatly affects her life and Red already had this concern)
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As for Sage, let's say Red solves this problem when this reunion occurs XD
Basically WN's "2nd Arc" is all based on imouto's situation and Red solving part of those past problems... including with Monk and Cleric from the party.