Ben told police he and his sister would regularly squabble about "silly little things" and said the row that morning had "been a 4 on a scale of 10".
Bethany's mother said the teenager had been happy and chatty the previous evening and they talked about how Bethany spent more time in her room, to which she'd answered: "Mum, I'm a teenager."
She described Bethany as a "reliable and capable" girl with a lot of common sense.
On the morning of her death on June 22 last year, the pair had argued because Ben had slept late and Bethany did not wake him.
During lessons later at Calder High School in Mytholmroyd, West Yorkshire, she doodled on her left arm in ink, "I hate my brother".
Bradford Coroner's Court heard she regularly scribbled on herself.
After leaving school, Bethany met her new boyfriend for a walk in the local park before catching the bus home to have her tea.
Bethany's accountant father Richard was still at work and her older brother had gone to the cinema with his girlfriend.
Her mother Estelle said Bethany took her dinner upstairs and played music.
She said when Ben returned home, he was asked to take a basket of laundry upstairs - he found his sister dead.
She was taken to Calderdale Royal Hospital and then transferred to Leeds General Infirmary and she died on June 25.
"I knew she had written something [on her arm] but I was not aware of what she had written," Ben told police.
Bethany's father Richard (pictured with his wife and daughter) said: 'I cannot understand why Beth has done this.' Photo / Facebook
Neither parent was called to the inquest but Bethany's uncle Dr Julian Wadsworth, a GP attended and said she was "making that difficult transition between being a child and being an adult".
Bethany's father said in a statement: "I cannot understand why Beth has done this. This is the last thing I ever expected to happen - it was out of the blue."
The inquest heard Bethany was a popular and happy girl who had a "typical relationship" with her brother.
He complained she talked too much and she thought he was "mean", family members said.
Her mother said she was "social media mad", regularly using Snapchat, Facebook, Instagram and Kick - and even moved her mattress into the spare room to be closer to the Wi-Fi router.
Richard Fitton said his daughter had loved pony riding as a youngster, adding she "still sucked her finger and slept with a comfort blanket" but fussed over hair and make-up like many other teenage girls.
Bethany's school record showed she was "good" and "outstanding" in many subjects.
Recording an open verdict, Assistant Coroner Peter Merchant said she left no suicide note and he could not be satisfied she intended to take her own life.
He said despite that morning's argument and the message on her arm, Bethany and Ben were normal siblings.
"It seems to me it was a typical relationship between a 13-year-old sister and an 18-year-old brother.
"He had slept in and she had not woken him up. He decided she should have done so and she took a view to the contrary.
"There is simply no explanation given to why Bethany would do what she did on that day."
