Owen was the cause of Ianto's problems, he always would be and he always has been. Ianto had been a rebel, because of Owen. He had started smoking, because of Owen. He hated his father, because of Owen. He was a werewolf, because of Owen.
If you had asked a 14 year old Ianto if he liked his half brother he would have said yes. If you asked a 17 year old Ianto if he liked him he would have blew his cigarette smoke into your face and told you to piss off.
Fights aren't uncommon between teenagers and adults but when one of the participants is a werewolf it is definitely not going to end well. It certainly didn't end well for Ianto Jones.
*FLASHBACK*
"Don't you dare call my father that again!" Ianto tackled Owen to the ground but the older boy flipped them over so he was on top.
"What?! A monster? It's what he is! It's what he made me be lucky he didn't make you one! Face it Ianto he knocked my mum up and left, why are you surprised he did it to you and Rhi?!"
Ianto kneed Owen in the stomach so the 16year old was on top and Owen was under Ianto again.
"You're right you know, you are a monster!" Ianto's fist connected with Owens face and despite the fact Owen was 3 years older than him Ianto still hit him with some force.
Time slowed down and Owens eyes turned misty, red smoke and fog seemed to fill up Ianto's bedroom as Owen gained power and with some momentum pushed the other boy onto the wall and sunk his teeth into his neck.
Yes Owen was the cause of Iantos problems always had been and always will be.