nygelgoode: Let's put it this way, if Nick's father, mother or sister decided to sing, they - I assure you - would sound so very, very different - in nearly everyway you can think of - than Nick. Now why should that be? Nick invented a "voice" which, to be, sounds artificial and very contrived. But if that's your cup of tea, fair enough."
There's a recording of Nick Drake singing All My Trials with his sister and their voices are very similar. Also, as mentioned above, the recording of Nick Drake's mum singing Poor Mum (maybe a precursor of Nick Drake's Poor Boy?) shows that he was hugely influenced by her singing.