I assume that you also do not want us to talk about relations with one’s spouse or parents.
I think that it is very hard to answer this question in a “one size fits all” way. That is because there are many variables at work. First, this depends to a great extent on the closeness of the familial relationship. For example, a person really ought to have a better relationship with their siblings than with their cousins. This closeness can also be expressed in geographic terms. In other words, a person ought to have a better relationship with their cousin who lived across town as they were growing up than with their cousin who lived 3,000 miles away. Finally, age enters into it in at least two ways. If you have a cousin who is 20 years younger than you, your relationship is likely to be less close than a relationship with a cousin your own age. Also, as you grow older and form your own family, your ties with your other relatives will, by necessity, weaken to some degree.
What all this means is that it is hard to answer this question in any generic way.
In general, we can say that a good relationship with your relatives is one in which you keep in touch to a degree that is appropriate for your relationship. In other words, you don’t need to talk to your cousins every day or even every month, but it would be good to be in touch with them on Facebook. But you probably ought to be in contact with your siblings much more. You also ought to have a relationship that allows you to interact easily when you do come in contact. That is, a good relationship with one’s relatives is one where there is not a lot of tension. You don’t have to think each of your relatives is perfect, but you should be able to get along well enough. Finally, you should have a relationship with relatives in which they are more important to you than casual friends. You should care more about them because they are your relatives.