OMG

i'm super confused with this lol could you explain the real thing? thanks

In korea they use "oppa" to call their brothers or boyfriends or close older men in general.
So what happened is .... mari kept talking about her oppa *boyfriend* to uke. One day she got a call from *oppa* but it wasn't next to a heart (such as this one♡) which means she saved her brothers phone number with only (oppa) name. On the other hand she saved her boyfriends number with oppa♡ . When uke saw her phone it was only (oppa) whos dialling so he thought her brother was her boyfriend.
I pray you got it ε=ε=(ノ≧∇≦)ノ

So the man she met exactly after uke saw (oppa) on dialling was her brother picking her up. Thats why he mixed him with her boyfriend

thanks! but old man is married? the boy already know mari's his sister in currently chapter?

Seme isn't married or in other relationships
the bold doctor is mari boyfriend and the one whobis married.