I anderstand the reason why people conceal the true about family,specially men. 

sometimes they do it,because they dont want to lost what they had built for years 

I mean their family,they feel afraid of getting separate from his family or end up in 

divorce.

So they decide to hide the true until to find the apropiate moment for saying it

for example 

I have a half brother 

my mother get married to my father even she knew he wasnt an easy man with 

wimen,she was not the only girfriend he had either,but as my mother was not jealous at

all.My father continued having a happy life, like he were singer.

there was a rumor in the neigborhood sound everywhere,but my mother did not 

care about it and my father had a baby in the stree with other marriage woman.Even my 

mother get upset she acepted the baby, but my father didnt  gave him a last name because

of his false father.

finally to tell you the true,the boy looks like my father,and everybody said the same.fourteen years later after his false father died,the boy end up eating and playing with us in our house,and now he is militar and he can not get separate from his five brothers.