"His siblings swim."
Translation:Hans søskende svømmer.
Sin, or sit if referring to a word that takes et instead of en (like barn), denotes that the object being "owned" belongs to someone previously referred to. E.g. Mette elsker sin søn means Mette loves her own son. Mette elsker hendes søn, on the other hand, means Mette loves some other woman's son. The problem is that, in a sentence like "her son is here", there's no owner previously mentioned in the sentence. So whether its Mette's son or not, it is translated as "hendes søn er her".
Hope that clears things up a little. If not, I'd be happy to go through it a little more comprehensively.