Unlike blood, water soon evaporates when it is spilt, and leaves no trace
afterwards. Figuratively ‘blood’ means ‘relationship’. When we say that
persons are of the same blood, we mean that they are descended from the same
ancestors. The interest we take in those not so close to us as blood relations
can be compared to water, which is thinner and less enduring. So says the
proverb, though the reality is not always in accord with it! ‘God gave us
relatives’, said the cynic. ‘Thank God we can choose our friends.’ Kinship,
however, is a very strong tie, and help is often given not for the sake of
individual but for the sake of the family.

‘I’ve never had much patience with that nephew of mine, but his mother is my
sister, so I paid the fine to keep the young fool out of prison.

An Albanian proverb with the exact same meaning says: ‘Blood doesn’t become
water.’