Better crush your hope now rather than later.
So... yes, she will. He is not her brother by blood. And he don’t think of her as his sister. He likes her in romantic way very quick. Like... after spending 1-2 years with her, he start liking her in romantic way.
While, she treat him as a kid that still need to eat a lot and drink milk to grow up.
I think their age difference is between 6-8 years apart.
Very soon, she will meet 2nd male lead who is about same age as Dietrich. He is cutie Tsundere. And in my opinion, Dietrich is a mix of Kuudere + Yandere.
Because she behaves like an older person...
He somehow feel the tingling in his heart, but still don’t have that adult romance feeling.
I think there was time skip or something, I don’t read the Korean. But it was when she is able to talk properly maybe around 3-5years old.
And this kind of development you can see in a lot of others stories especially BL.
Like, one is older like around 8 years old, when the other one is just a newborn baby.
But the 8years old somehow felt a strong need to treasures and protect the younger one.
As they grew up, those feelings can develop either into family’s affection or into romantic affection.

Please don’t get with your brother