Her elder brother, Prince George, aged four, will be a page boy alongside three other young boys.
So far, the details of the bridesmaids' dresses and the page boys' uniforms remain under wraps, the BBC reported.
Markle, 36, will not have a maid of honour as she wanted to avoid choosing just one of her closest friends, it said.
Merkle is a humanitarian activist and former American actress.
As well as his niece and nephew, Prince Harry has also picked three godchildren - three-year-old Florence van Cutsem, two year-old Zalie Warren and Jasper Dyer, six - to have starring roles on his big day.
Markle's goddaughters, Remi Litt, six, and her elder sister, Rylan, seven, will also be joining the procession of bridesmaids.
Prince George, the son of the Duke and Duchess of Cambridge, will have experience to lean on - he was page boy at his aunt Pippa Middleton's wedding to James Matthews last year.
His sister, Charlotte, recently showed she was undaunted by the limelight when she visited her little brother, Prince Louis, at hospital after his birth.
With the world's cameras trained on Charlotte, she seemed to enjoy the attention, waving sweetly and smiling at the bank of photographers, the report said.
The announcement on bridesmaids comes during a turbulent week for the royal couple. After first telling TMZ he would not be flying to the UK to walk his daughter Meghan down the aisle as planned, Thomas Markle later said he had changed his mind, according to the US-based celebrity news site.
"I hate the idea of missing one of the greatest moments in history and walking my daughter down the aisle," he said yesterday, according to TMZ.
Markle has been involved in controversy since it was revealed he allegedly staged a series of photos captured by an American paparazzo.
Just hours later, the news site reported that Markle would not be attending as he said he would be undergoing heart surgery today, just three days before the royal wedding.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)