Ta muchly Sirusrocks.You are absolutely right.

I went exploring and found that JKR explains this half blood shinannigans as being like the Nazi's 'Ayrian Race' obsession. So yeah, it's not just your parents, it's your heritage. Like the Nazi's, the Deatheaters kind of make it up as they go: if they 'like' you- ie. if they are afraid of you, need you or want you to be one of them - they might not look back too far but if they hate you (a la Harry) you're blood can be seen as 'tainted' very quickly. Harry is def. a half blood.

JKR goes on to say that the purety of blood has become such an obsession in some families that any muggle or squib gets pruned off the family tree (Blacks being obvious eg.). She says "This topic is dealt with in HBP"!! What do you reckon this means? Maybe someone doesn't know they're related 'cause their parents were pruned?

We know Mrs Weasley is related to Blacks and therefore Malfoys. Does this mean Ron and Draco are second cousins?

We don't know anything about Harry's Father's except that they are an old wizarding family. Are Harry and Ron related? Harry and Draco related??

Voldemort's Mum was a witch but we don't know much about her family either. Are Harry and Voldemort related?