Make Nagi Erika's long lost brother, make Sachi snap out of it and realize that Nagi is her brother also and stop thinking weird things, have Nagi being Hiro's mystery fiance and we are all in the clear.
I only see this ending in 2 possible ways:
1. He does have a twin brother and he's just somewhere who knows where doing whatever
2. At some point in life they figured out that the switch happened but decided not to do anything until Nagi grows up humble and can take over the company while marrying Erika/Sachi(That way the 2 families don't have to separate) and Umino just forgot or had an accident/amnesia scenario
Honestly, the only good option here is Erika. Sachi has a horrible personality, she only thinks about herself and is very obnoxious most of the time, and how the fuck does she suddenly like him romanticaly when they've been siblings their whole lives, japs are weird man. And Hiro is even worse, saying she's engaged and whatnot but still keeping him interested enough in her to not go with the other two; she even says to Erika that she's wants to take him for herself, literally a manipulative
hoe
. I think I'm pretty much done with the series, shouldn't even be a harem, author didn't need to bring the little sister into it. And no I'm not typing this because I'm some Erika fan, I just think it's the best option considering their chemistry together and how it's a balanced relationship with how both of them care for eachother equally.