I think it's because she was the only girl in the family except the mom of course but also because since shes family now, she knows them more personally and maybe the brothers dont really have a girl that they are personally close with so they fell for her? I dunno. It's just a guess from basic manga story
I think this is actually a otome game and it was a anime before this manga
Well, to answere this, there's nothing really amazing about her, apart for the fact that she is the protagonist of a reverse harem manga lol
So, yeah, they all have to fall for her
*answer
What's so good about Ema.? I'm pretty sure the reason why they were so hung up on her was because she led them on.. like with Subaru and his basketball issues.