
Like the author did not want us to hate the omega brother so they came up with dumbass excuse. That way if (or when) the spinoff about the brother is created, it will be supported.
Sorry sis, the only way I could swallow that dumb excuse if there were at least three pages dedicated to how much an idiot the younger brother is.

We are seeing things from MC's perspective.
Remember he was a twin, but when we saw his brother, his brother in MC's memories did not have a face. I think everyone having the same face is a manifestation of MC's guilt that he has towards his brother.
Either you don't have a face or he's looking at his own face (ie his brother's).
Hiro-kun is just going to end up marrying her just to make sure she doesn't find a worse man (╯°Д °)╯╧╧