When? Where? I don't remember ever seeing an elven brother/sister or templar. Or reading about it in any of the codexs or books.
The female elf you save from having molten gold poured over her in Sebastian's Act 2 quest writes you in Act 3 thanking you and tells you that she is a Sister in another Chantry. It even has Sister in front of her name where she signs.
As for them being able to be Templars,
@Maugrimm Elves aren't barred from becoming templars. They're just not very common. Evangeline in Asunder was almost an elf, actually.