I like emmaloo's. I agree that there needs to be a reference to the main character(s), since their dilemma and struggles are likely to be the primary focus of interest.
'[MC1] and [MC2]'s love is threatened by family duty and racial prejudice. Is there a way to bridge the divide?'
What's missing from this, of course, is any mention of the time in which the story's set and the multiple locations. But with a strict 20-word pitch, apparently, you seem to have to leave out all perpheral detail (no matter how important it seems) and cut right to the conflict at the heart of the story.