well it doesn't say is outright? Kinda? When Soga open the envelope towards the end and then crumples it when Haru comes in, he was looking at the blood test results.
In the previous chapter when talking to the secretary he had said "so just a hair sample will do?" meaning he intended to have a blood test done.
Also when he said he dad was hospitalized, he considered telling Haru because it would make him happy to have his father back but decided not to rock the boat.
...I'm sorry, I'm a bit slow... does that mean they're biological brothers? ;;
Yes. They have the same father. Different mothers.
Aah. Thank you for clearing that up for me T_T
Ahh. I thought they weren't real brothers but scrolling through the comments it seems I was wrong... e_e
I have no problem with it but what I /do/ have a problem with is the fact that Tatsuya didn't tell Haru in the end... relationships are all built on trust, y'know?!
...I guess he really doesn't know ._.
On a sidenote http://www.mangago.me/read-manga/uso_to_kiss/mf/v01/c001/33/
I can relate, random dude. I can relate xD