As relationships become more distant, the practicalities of testing intermediate relatives reduce, shared inherited DNA becomes more dilute and so the utility of the standard approach to relationship testing using STRs testing diminishes.
STRs may still be useful for testing uncles or aunts (avuncular testing) especially if samples from a parent and grandparent or two are available but when we get out as far as cousins it has all but lost its ability to be informative without testing many intermediates such as parents, grandparents uncles and aunts.
However, we can still use mtDNA and/or Y-STRs to check uncles, aunts, cousins, second cousins and so on who we think are related to us via unbroken female or male lineages. Also there is a relatively recently developed SNP method that uses a different sort of approach to give a measure of how closely two people are related. The SNP method is great in that it is informative about all types of relationships out as far as third and even fourth cousins. It gives you a good clue if a genuine blood relationship exists.