An apartment in the popular resort of Kusadasi became a second home.
But on 18 August 2011, a day out ended in their murder.
The bodies of both women were found in a forest just outside Izmir - the country's third largest city.
At the heart of this tragedy is Marion's daughter Shannon.
She was only 15 at the time.
Not only did she have to cope with the enormity of her loss alone, until relatives flew out to bring her back home, but it was her boyfriend Recep Cetin who quickly emerged as the chief suspect.
Not long after, his father Eyup was also implicated and both were later charged with the double murder.
Last year, Shannon spoke for the first time about the loss of her mother.
"She was my shadow ... It is just like different, because everything is different because she is not here now. It is just hard to believe what actually happened,'' she told me.
We did everything together.
Shannon Graham, Marion’s daughter
The reality is we may never know exactly what happened that day.
This is what we do know.
Recep Cetin paid for Shannon to go on a boat trip with a friend.
He took Marion and Cathy out for a day's shopping in Izmir. But something obviously went badly wrong on that journey.
Did an enraged Recep turn on both women because Marion was about to put an end to his dreams of marrying Shannon?
That's what both families firmly believe.
And it is what Recep Cetin is reported to have told police in an early interview.
We may never know for sure - partly because Cetin has changed his story so many times.
He claimed Marion and Cathy were abducted by the local mafia.
Then he confessed to the murders, but later said he acted in self-defence.
His father Eyup Cetin has always denied any involvement.
And his son has repeatedly told the court in Ismir that his father wasn't there.
But a key witness has placed both at the scene.
The trial of this father and son is very different to the system here.
There is no jury - four judges will decide their fate - and all the evidence is weighed up in a series of hearings lasting months.
And this case has taken may twists and turns - some of them bizarre.
Recep Cetin lied about his age to avoid standing trial in an adult court.
Bone marrow tests were ordered - he was 22, not 17 as he first claimed.
And when the families flew out to Ismir to see the father and son in court together for the first time, that didn't happen.
Eyup Cetin was the only one there and his son was still in his prison cell.
Marion and Cathy's relatives flew home furious that such a mistake had been made.
But it wasn't a one-off.
Both families are back in Turkey. They have been told this case will conclude tomorrow. And they hope that this time – the outcome – will help them finally find closure.
Sharon O’Neill, UTV, in Turkey
Just recently, Recep Cetin claimed he was mentally ill.
The authorities ordered tests, but they were carried out on the wrong man - his father.
All of this has been very difficult for two families from Co Down, desperate to find the truth.
Cathy's brother George, speaking to me in Izmir last year, summed up his frustration.
"I can't understand why, there is no reason you know. I have a lot of questions - just where do you start? But it is in the hands of the court and they will sort it out,'' he said.