"Twins have the same parents."
Translation:Ĝemeloj havas la samajn gepatrojn.
I don't know exactly, but probably for the same reason it's necessary in English. "Twins have same parents" doesn't sound right to me. I suspect it has something to do with la/the being a definite article, meaning that it defines a specific thing. http://www.edufind.com/english-grammar/definite-article/
I am reading this - two years after you posted it!
I would answer something like this. Can we talk about having "a same parent"? Can we talk about having "some same parents"? We say "the same parents because it's a definite set of parents. The parents of one are the parents of the other. Notice that I said the parents in both cases in the last sentence.
It's similar in Esperanto.
I do think there are cases where we wouldn't say "la sama" in Esperanto even where we would say "the same" in English. Off the top of my head, I would say the rule is that we don't use "la" when using the adverb to describe things that are the same.
- Mi havas la saman ideon - it's one idea, the idea.
- Mia ideo estas sama kiel la via. There are two ideas and they are the same.