"Sioned is the tallest."
Translation:Sioned yw'r dala.
Afraid so, for a couple of reasons:
Sentences which have the superlative (-est) describing the subject (Sioned) are always (almost) emphatic in Welsh, so the thing being emphasised (Sioned) is put at the front of the sentence - Sioned yw'r dala. This emphatic construction is much more commonly used in Welsh than in English.
Mae Sioned yn dal - Sioned is tall (note the need for yn in yn dal to link Sioned and t/dal)