At least they say at the start of the comic that they are brought in as wards and possible future spouses and not as siblings... guess we're just fed up with the siblings to lovers trope...
And the main problem... is that ml looks too much like the father... including age... mc looks like a child and he looks like a full on adult... so... it looks pedo af... because it feels he feels in love with her even with her being a kid...
The problem with your thinking is that the the grown up seeing in these comics entail: a- them usually being adopted siblings and not just childhood friends; b- mc is more often than not an adult inside while ml is not, aka grooming (since when aduldts she falls for them cuz they've became the "perfect husband material). Honestly is rare to see the mc being bothered by their actual age compared to ml... and even rarer to see them acting their real age... I'm kinda tired of seeing adult women in bodies of kids/teens acting not childish but naively as if they have knowledge but not memories and experiences of their og lives...
They look like father and daughter!!! How are u supposed to spin a romance from this bro