For a word that can refer to both married people or people in a relationship, you can use cheat, cheater, two-time (informal). Two-time is also used as verb, as reported from the NOAD: He was two-timing a fiancé back in England.
For people who are married, you can use adulterer (which is gender-neutral) or adulteress (when referring to her).
Nepotism is quite a specific word for this, but maybe is too strong for your needs. You could call the person a nepotist.
noun
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
Dictionary.com, "nepotism," in Dictionary.com Unabridged. Random House, Inc. Accessed: July 09, 2014.
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).
Best Answer
I think the word you may be looking for is aloof, meaning "not friendly or forthcoming; cool and distant". [Websters]
Edited to add: The word also carries the connotation of supercilious, which means "behaving or looking as though one thinks one is superior to others".
You could also simply say: