Note that Yui's inner monologue effectively said at least twice that she isn't jealous, and she at least claimed that she doesn't really care about attention--she wants to do the best she can as the person she is, and if that means being overshadowed by someone else who's doing their best, she doesn't mind.
In any case, big brother is off at a fancy university in another city. You won't see him again for quite a while (maybe a month of in-quest time). The reputation left behind by the city's previous magical girl? That's the shadow she'll be in now. And all you really know about her is that she held the territory for "years", mentored a number of other magical girls, and stuff like that. Her wish, her powers, her personality, who knows? Well, Kyubey, I guess.
Incidentally, if you want to put off the wish and interrogate the bunnycat some more that's also fine.