A beautiful picture of this happy couple. You managed to paint this side view very well, and while this kind of portrait of a happy loving couple is nothing original, I don't think I have seen it with Tuor and Idril before. And I think it is quite appropriate for them, their pose and the tender gesture of hands really express their mutual love.
As for the technique, I like how you painted their clothing in a realistic way (that is always a challenge for me personally) and especially Idril's hair is beautifully shining. In my opinion the colours are maybe too light, it could use more contrast and deeper shadows. On the other hand, as you yourself mentioned, this style makes it resemble fairy-tale illustrations, which is not a bad thing - it is a possible way to illustrate Tolkien's stories (especially those happy ones. I grant it might be odd to depict, say, Turambar and Nienor in such style).
Also, great work with the ornamental design, it frames the portrait very well and the overhanging greenery is a nice touch.
Overall, a great portrait of one of my favourite couples from Tolkien's legendarium.