almost didnt read the text and thought you were asking about onimai itself

I think it breathes some realism into the setting. It may just be but in my experience siblings often have complimentary names. It doesn't happen in every family but it happens enough that it's noticeable. My aunts/uncles on one side all share the same initials. There was a girl I knew in school, Faith, that had a younger sister named Hope. They were the youngest of 7 and everyone of them had some name like Faith or Hope; only remember theirs because they were in the same grade and a grade below me, respectively.
Maybe authors give their sibling character similar names to symbolize how close the characters are?

I like it because it's easier to remember names and relationships when there's a base pattern or theme to them.

what i actually keep mixing up are mahiro from onimai and mashiro from hirogaru sky precure

It's not something that bothers me too much. I can't remember names of characters in shows 99% of the time so it doesn't matter. In theory I'd want names to be very different and distinct, but that just doesn't help unless I feel very invested (like I can remember some video game rosters from 20 years ago)

>>2937
It's funny you post that image because I get Risa mixed up with Rika, but have never gotten Rika mixed up with Mika. Is it just that the change is in a less prominent syllable or is it easier to differentiate when there's an established relationship between them like being family?