Recently I saw a BBC documentary on a family from a remote village in turkey where one couple has 18 children and out of which 5 are walking on four legs. These children are 28-34 yr old & have been like this since the birth. Is that story true ? if yes, then what is the explanation behind this in terms of biological evolution ?
It sounds like this might have been the BBC2 documentary "The Family That Walks on All Fours", in which case the Wikipedia page about it (linked) may contain all the answers you need.
In short: yes it's real; this is the Ulas family in rural Turkey. This is thought to be the result of a an inherited recessive genetic mutation resulting in cerebellar ataxia that impairs balance, meaning that they cannot balance well enough to walk confidently on two legs. From the Wikipedia page on the documentary:
Not being able to manage the balance needed for bipedal walking, they perfected in its place their initial bear-crawl into an adult quadruped gait. The family's walking likely has nothing to do with genes involved in the human evolution of upright walk.
(a "bear crawl") is a four-legged walk which uses your hind feet, rather than your knees)
It does not tell us much about evolution, but it does tell us something about how plastic motor development is. However, several of the experts interviewed in the documentary became interested in the case because they wondered if it might in fact offer insights into the genetic basis of bipedalism, so it's not a ridiculous assumption.