Because I want to laugh and giggle with you, Big Sis...! A more literal translation would be "giggle and tee-hee", or "giggle and titter" but both of those sound so... odd. キャッキャ is the sound made when someone is laughing happily or excitedly, used especially when referring to children. ウフフ is a quiet laugh that can be either slightly cutesy or yandere-cutesy.

...Huh?

Wh-what's with that reaction!?

Fine then! You idiot! Stupid! Nincompoop! I hate you, Big Sis!

N-nincompoop!!? --hey, Flan!