I know one guy who always helps his own family and relatives but he never ever helps his friends. How to call that person ?
Nepotism is quite a specific word for this, but maybe is too strong for your needs. You could call the person a nepotist.
patronage bestowed or favoritism shown on the basis of family relationship, as in business and politics: She was accused of nepotism when she made her nephew an officer of the firm.
Origin: 1655–65; < Italian nepotismo. See nephew, -ism
There is also the phrase Blood is thicker than water which means family is more important than anyone else. This is perhaps a softer term impressing the values of looking after the family over the needs of friends but doesn't provide a term that you can use to call someone directly.
In modern society, the proverb "blood is thicker than water" is used to imply that family ties (blood) are always more important (thicker) than the ties you make among friends (water). It generally means that the bonds of family and common ancestry are stronger than the bonds between unrelated people (such as friendship).
Wikipedia contributors, "Blood is thicker than water," Wikipedia, The Free Encyclopedia, (accessed July 9, 2014).
"Nepotism" is good: I think you can say he is nepotistic because this is the adjective.