I think this would be unlikely to happen, if the children were raised together from *early* childhood. Children who are raised together are hardly ever attracted to one another as adults (as people in Kibbutzim discovered). It's more likely if the teens were older children when they became family. If it happened, then, in principle, I would have no problem with the teens dating, since they are not related by blood.
Having said that, there could be complications if the relationship turns sour. If I were in that position, I would want to have a conversation with both teens, individually and together, about how it might affect their relationship as siblings/friends, and how it might affect other relationships within the family. They would need to bear in mind that maintaining a long-term friendship between them is more important than hurt feelings or temporary anger, and to understand that there is a possibility that their romance may not last long-term, and they need to behave towards one another in a way that means they can remain friendly afterwards if it doesn't. They would also have to agree to never ask siblings or parents to 'take sides' between them, and to develop the skills to address issues that arise between them (and they need to understand that issues *will* arise between them) on their merits, rather than descending to personal attacks on one another. It will not be possible for parents to help them through the issues if they are bent on personal attacks on one another.
I would also want to see a conversation between the biological mothers of both of the teens, in which they discuss how they can remain genuinely neutral in any issues between the teens. Taking sides with your own child against the other is a sure way to create problems in the relationships between the adults. Both mothers need to be able to view the teens' problems dispassionately, and respond even-handedly to the *issues* rather than from a desire to 'stand up for' their biological child. This would necessitate a great deal of trust between the adults; trust that your biological child's well-being is as important to your partner as that of their own biological child. Parents are notoriously reluctant to hear any criticism of their child from others, so this may be a tall order. But if the parents cannot do this, then they are endangering their own relationship.
Ultimately, I do not think that the teens should be expected to deny themselves a relationship in the interests of protecting their parents' relationship. Parents are adults, and should be capable of protecting their own relationships. It is not the responsibility of their children to do so. It is up to the adults in this situation to be, well, *adults* and to understand that they are going to have to work hard on any pre-existing issues they have regarding trust and defensiveness within their relationship. Looked at in a positive light, it is a great growth opportunity for the individual adults and for their relationship.