The sentence should be: राज और नेहा भाई-बहन हैं ---as it is in the original sentence, the verb is singular although the sentence is referring to two people, not one.
Just for fun:
If you say नेहा और राज, should you then also switch to बहन-भाई, or is भाई-बहन really treated as the word for siblings?
It would still be भाई-बहन. That's the word for 'siblings' like माता-पिता/ माँ-बाप is 'parents' and दादा-दादी is (paternal) grandparents.