You mean... you don't see anything special about her!? ... 😵 (Just kidding)
Nothing you should know, really. This is not the result of some weird experiment or something. Their breasts are just this big. Never really stopped growing at the point where it stops for other women.
They are different. But then again, everyone is different. You could call them mutant or deformed... but... that might offend them. Other than that (as you might see in time) there's not steady story-line or even much consistency in this series. It's a satire. In one episode Bianca is a playable MMO character, in another her sister is sort of a Jedi Knight in Starwars, a student in another... it's all quite random... but this series is not created to make much sense. As I called it at first, it's a bit wacky.