Sioned yw'r dala
Welsh uses the superlative form of the adjective for both 'the ...er' of two and 'the ...est' of three or more.
You see this a lot in place and houses names such as Ninfa Uchaf (Upper Ninfa) or Tŷ Isaf (Lower House)
In Welsh, unlike English, we use the superlative for comparing two or more, so the above could be translated as either 'the tallest' or 'the taller'.
However if we want to use the comparative, eg 'Sioned is taller than Siân' we say
Mae Sioned yn dalach na Siân
Diolch am eich sylwadau. The translation 'Sioned is the taller' should then be considered correct?