In August 2013, Irish teenager Ibrahim Halawa was arrested, along with hundreds of others, during protests in al-Fateh mosque for allegedly contributing to violence during protests in Cairo - charges that he and other witnesses deny.

After more than three years in an Egyptian prison, his trial was due to start on Tuesday. His sister, Somaia, wrote this letter to Ibrahim to express the family's pain over his ongoing ordeal.

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Dear Ibrahim,

It has taken a great amount of time to reach this stage. It has been three years, five months and one day since the day you were put away.

For some, that might not mean much; for others, it could mean a new job, a newfound love, a wedding, a newborn child, a newfound interest, or a graduate degree earned.

But unfortunately for you, my brother, this has not been the case. For more than 1,200 days now, all that you have experienced is one pain after another, one abuse after another, one disappointment after another, one unjust moment after another and, simply put, one heartbreaking memory after another.

Both your mental and physical health continue to be affected, up until this day. Having to hear that your younger brother was suicidal, and that there was nothing that you could do to help; seeing your closest friends graduate, start working, fall in love; missing your siblings' graduations, weddings, baby births.

We can only imagine the frustration and the pain you must be feeling while trapped behind bars. There is no doubt about your innocence. I am 100 percent sure of it, as sure as I am alive today.

We all understand exactly what freedom and independence means. Just a couple of months ago, we celebrated the 100th anniversary of the 1916 Irish Easter Rising. These values of freedom and principles of independence are the ones that you grew up with, instilled upon yourself. Why else would a boy, just 17 on the date of your arrest, endure all of this?

You might think that for us, the suffering gets easier as days go by - but instead, the agony increases more and more each day. My own brother has become a dream to which I wake up, wondering if he will ever become reality again.

What makes it harder is the memories that we had built, growing up together around Dublin in our primary schools that our nieces and nephews now attend, or even in our home where we have lived for almost two decades.

It is not only these distant memories that intensify our agony each day. It is also the memories now, and the ones we will form in the future - the ones in which you will be forever missed. The ones we can never get back.