Friendship is very important to marriage; a marriage is so much stronger when you like and respect your spouse. If you mean friendship with people related by marriage, such as in laws, good...
That's easy all you have to do is be nice to that person and make him want to stay with you no matter what. But don't show that person what your doing because that would just scare them off.
All cousins are related. Those who are related by marriage are not biologically related so developing a relationship with them is only a matter of convention.
However, even marriage between first...