Because Mama-chan is overprotective and Chiaki-chan is totally precious and and cute and stuff and Mama-chan is worried that Ibuki might hurt Chiaki-chan's feelings or something but Ibuki assured Mama-chan that Ibuki would never do that because if something bad happened then Ibuki would be sad too right so there's no way Ibuki would wanna do that right?

[... that sure was a run on sentence.]