I really don´t get it, Casper.... if you agree HERE that stesøsken = stepsiblings, then why do you insist on translating it to "adoptive" in the Spanish version??
In English we could probably say half sibling or maybe we could even say step-sibling, which is the same. However, the most correct way would be to say step brothers and sisters, or half brothers and sisters. If the names in the translation are female then we would say "I have three half sisters", and if male "I have three half brothers." And if both "I have three half brothers and sisters". And you can replace the world "half" with "Step".