its quite simple michishio, Yuudachi is shigure's sister! it would be rude to not talk to your own sister! even if she cant understand a word she says.
"She don't know talking good like me and you so her vocabulistics is limited to 'Poi!', 'Poi~', and 'Poooooi!'. Exclusively in that order."
Eh? What?Pooooi!!Poooi?Do you even understand what she's saying?I didn't understand a word!Then why did you even talk with her!?Whaaa?Poi-poi!Poooi-poi♪Poiii♪