It's usual for the hospital to ask a patient for details of next of kin on their interminable forms but, if this was an emergency admission that probably hasn't been done. In a legal sense I think that everyone would expect it to be a blood relative but I'm not sure about that. Have you had a word with the Citizens' Advice Bureau?

Best wishes to your father in law - hope all goes well. When he recovers it may be something to talk to him about.