[Bothersome isn't quite the adjective Iri wants to hear applied to her daughter, but truth be told, she's learned just how unruly Illya has become over the years, and Rin's assessment isn't exactly untrue, so...]

I appreciate that you want to take care of her, but it might be for the best that you leave her to me. She's a very... headstrong child.

[And nothing will end well if they argue, as Iri suspects has already happened.]