Marc Thompson said the "hardest thing is not knowing" where his twin brother Ben is.
The 34-year-old from Haverfordwest has been missing since Saturday, after he watched Wales' Six Nations game at the Millennium Stadium.
He told our Correspondent Carole Green that this would be "very out of character" for him not to contact his family.
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An ancient grotto in Pontypool made from shells, bones and the teeth of animals will open to visitors for one day.
Events are being held to celebrate the 150th anniversary of the Welsh colony settling in Patagonia.
Chris Coleman says is would be a 'big blow' if Wales were to lose the three points they won against Israel two months ago.