I want to eat the banana bread my sister baked for her colleague
Why do you want to eat the banana bread?
I think it will be delicious and I want to eat it.
It's good that you think your sister's baking is good but she made that food for her colleague. It would be wrong to eat it because it doesn't belong to you. You should try to get her to make another one for you if you want it that badly.
I don't see why she can't share it.
So, this bread is not yours and your sister has already promised it to someone else. It's improper for you to eat what belongs to someone else.