It's called taking the high road for a reason- it always works out better for you in the end and you will end up in a better, higher place in life than they will. One should be very cautious in ever intentionally burning bridges with someone, even if you can't imagine a situation that it could hurt you. People "with issues" always get theirs in the end.
To be successful in life (by whatever definition you want to use), you don't have to be great at anything, you just have to not be self-destructive and that puts you in better shape than 80% of the people out there. How many people destroy or at least continually set back their life with stupid decisions - ones that result in divorce, loss of jobs, and even with ones that cause them to lose their moral compass or sense of priorities (I put animal rights, goofy religions, etc in this category).
Do you think your cousin is truly happy? People that aren't want to make reassure themselves that everyone they come in contact with is as miserable as they are. That's why its dangerous to consort with pessimists. The greatest gift my wife has given me is helping me learn to be happy with where I'm at and what I'm doing - not to put too much emphasis on tomorrow or "someday". I grew up with family members that always worried about "what was wrong".
It doesn't hardly ever hurt us to be polite. You never know when it might unexpectedly help. My pesonal exception is when it involves someone potentially doing something detrimental to my kids. That's when it doesn't hurt to let people know you have "fangs" that can be used when appropriate.
