The Northern Irish family from Collegelands on the Armagh/Tyrone border came together on Sunday to mark their entry into this year's Guinness Book of World Records at their childhood home.
Although a day of celebration, the gathering was also an emotional occasion, as the idea to register the siblings for the record was that of the family's youngest sibling, Austin Donnelly, who sadly died in 2015.
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"At my sister Maureen's 90th birthday, my twin brother Austin said we must be the oldest family in the world. It was a fly-away remark but it grew legs and we began making enquiries into registering for the Guinness World Record.
"We lost Austin on Christmas Day in 2015 and after that there was even more of a determination to get ourselves verified and into that book, in his memory.
"After Austin died, it took a number of years off what we needed to make that record, but after 13 months we made it up again.
"It was a great day on Sunday, we came together from all over, Mairead came from Coventry and then some of us are in Dublin, Belfast, and then from all over Northern Ireland.
"It was an emotional day as well because Austin wasn't there to celebrate with us. I could hardly make my little speech there was a lump in my throat," said Leo.
The siblings are the children of Peter and Ellen Donnelly, who welcomed 16 children throughout their marriage. While Leo said his dad "died young" at 79, his mum Ellen Mullen lived well into her 90s. The parents tragically lost their youngest son Michael (25) in 1975 following a car accident, while Oliver passed away at age 64 after a battle with cancer.
Despite fathering 16 children, Leo's father Peter had been an only child, while his mother Ellen was one of 14. Peter was a very successful farmer and entrepreneur, with a successful apple empire. The Armagh man was one of the first people in Northern Ireland to have a tractor, and to secure a license to export apples and potatoes to the UK.
Peter's legacy lives on, says Leo, in his family, which has grown and grown to make up almost 200 grandchildren and great grandchildren.
However, Sunday's occasion was one solely for the 13 siblings, celebrated at their family home, where Leo lives still.
"I'm not sure we'll all ever be able to get together again, we're all getting old, Sean is 93 now! All 13 of us are pensioners.
"There were no wives or husbands involved in Sunday, just the brothers and sisters. We had a beautiful cake and the Guinness adjudicator flew over to verify that everything was in order."
Leo said it is a great achievement that the family will now be included in the annual Guinness Book of World Records next year.
"We're going to get in the book next year. When we started this thing there was a determination to bring it all the way and it's great now to have that title. It's more important to us because it is in memory of Austin."
The siblings were the subject of an BBC NI documentary, The World's Oldest Family, due to air once again in June however, Leo said the process of getting to where they are now was an arduous one.
"We had to get everyone's birth certs together and of course some people didn't know where theirs was. We had to get some of them sent out again, and then the marriage certs for the girls who changed their names. We had to have three referees back us up of course, to say we were who we said, including the local priest!"
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