There are some great lectures on YouTube from the best universities in the US. One is an entire course about Human Behavioral Biology by Robert Sapolsky from Stanford. It's close to 40h, so I don't remember exactly where he said it, but we humans have some excellent built-in mechanisms for kin selection. For example, by using smell, we can subconsciously evaluate how much genetic material we share with a given individual. The look is also vital. The closer we are related, the more we care about a given person and the stronger the attraction is. We have mechanisms to prevent this attraction from becoming sexual, but most do their job when we are young. For example, we perceive a Family with whom we grew up as a part of ourselves, and therefore, there is no sexual attraction, only caring.
They never grew up together, so the most important natural protection has been completely disabled.
On the other hand, Anthropology teaches that only two traditions are shared across all cultures.
The first one is about treating dead relatives with respect. Respect means whatever given culture thinks is appropriate: burning, burying, eating, embalming - whatever.
The second one is about incest. It's taboo. Always. God/gods don't like it when you want to have a little too much fun with your brother or mother. It is additional protection against the situation you've encountered. Guard that for every culture on the planet was more important than anything else, as cultures that did not have this feature were extinct so fast we've never heard about them(some rulers did their best to break it, but a taboo that has to be broken existed). So this taboo seems very much a must despite some built-in protection humans already have.
In the case you've described, they are only half-siblings, so it's blurry, and additionally, there is no peer pressure as they are too old for this, and nobody cares.
So unfortunately, there is a powerful attraction between them, but the most mechanisms that should prevent that are not available, and you are the only source of peer pressure. In that case, you should provide extreme pressure and be prepared for plenty of resistance. They probably feel like that was love at first sight (probably rather smell, to be precise), and both will see you as an enemy who stands in the way of their "obviously platonic" love.
[This message edited by Stich at 11:21 PM, Tuesday, February 7th]