"Eleri Lingo is my sister."
Translation:Eleri Lingo yw fy chwaer i.
why does this sentence use yw rather than mae, and why doesn't it start with the verb?
This is an emphatic sentence, which is the usual construction to use with names, jobs and roles - here Eleri is being emphasised.
The thing being emphasised is moved to the front of the sentence, displacing the verb from its usual position.
If the thing being emphasised is a noun (including a name) or an adjective then it is follwed by ydy/yw rather than by mae.