... However it does not appear that FH will allow you to enter siblings without a parent ( I guess this is something to do with family groups) so the only way I can see to do it is to enter a dummy parent called unknown unknown ( if mother) or unknown surname ( if father) ...
Some people take against this idea because they believe it to be misleading. I don't think I'm going to confuse myself by having someone called "unknown unknown", so I don't have a problem with such a name.
It becomes more of a problem if you transfer the data to someone else - particularly if you cross a language boundary. Imagine trying to trace a German family with parents Herr and Frau Unbekannt if you didn't realise that Unbekannt meant Unknown....
Personally, I prefer to create a dummy person because I can add notes to that person saying what the current state of research is for them. Again, there are other ways of doing this.