Oh wow, the violent younger sister has more sense than the working adult. At least, a lot more eloquent. On a more serious note.
I have to echo some of ronasokily's points about the violence in this series. The vibes I get from Mikanuji is we are only supposed to treat the violence from Minami's parents and that one bit involving Shizuku 'kidnapping' Minami as actual abuse. Abuse that carries physical AND emotional scars. The physical smacking Iori does to Minami? Mikanuji's take on the decade old tropes of lovers' quarrel in anime. It's usually very one sided. There will be one crazy ass mofo who takes out her frustration on the more masculine partner. Emphasis on masculine, because people's perception that the more masculine types can handle abuse because... social expectations. Usually such lovers' spats and physical violence towards each other are greatly exaggerated and cartoony so you know "oh, it's not serious. They'll make up by the end of the episode/mini arc." If the emotional cues that prompt such acts of violence afterward are apologetic (rare), typical tsundere shit (common), the abused character venting to friends/others about relationship woes (common,) followed up with gags (also common, such as the KFC + sushi combo), and whatever other things I can't remember right now. It's considered "safe." It's "safe" because it's resolved. It's "safe" because the author didn't intend for this to be taken seriously. It's only serious when the mood after the fight is followed up with a tense BGM, or the abuser being sad/regretful. The author wanted to evoke that "teehee slightly violent couples's spat" trope, but due to probably cultural gap or just a terrible way of portraying the thing, a good chunk of readers here react with "OMG, DOMESTIC ABUSE!" ronasokily mentioned telenova kind of feel. Yeah, that's pretty much it. Japanese TV shows also adopted a very anime approach to their acting as well. Everything becomes greatly exaggerated. Iori overreacting to Minami's insecurity by trying to smack her out of it? *One of the oldest anime trope ever. See tvtropes for 'Get a Hold of Yourself, Man!' Yes, that mouthful is what it's called. (Too lazy to look up formatting guide for this. Google this yourself)
If you look at other examples of violence should not be taken that seriously in the Mikanuji's universe when it involves close friends/couples, check out the younger sister and Shizuku combo. "But Minami is hurt enough that she needs to wear bandaids on her face!" We are still meant to take it as "it's not that serious. Just a typical anime romcom moment."
"But-"
Anime. Romcom.
TL;DR version. Mikanujij didn't intend for us to see this as DV.