For some reason, this trope is everywhere. There’s that one uncle (or aunt) that is just utterly evil, and heaven forbid your parents die, because you’re guaranteed to have to live with the dickhead. We see this trope appearing with the Dursleys in Harry Potter, Aunts Sponge and Spiker from James and the Giant Peach, and going all the way back to Shakespeare with Claudius in Hamlet. For one reason or another, writers are big on evil uncles, and if you think I’m exaggerating, look at this list on TV Tropes.
Why this can be bad: As a reader, I’ve come to expect one of two things from aunts and uncles: they’re either super evil or super cool (aunts, however, are rarely cool, and almost always fall in the “annoying” or “evil” categories). Usually, though, uncles are the former. And why? I’ll bet money on it having something to do with succession. Perhaps we have Shakespeare to blame for this because of Hamlet and Richard III, but whatever the reason, we’ve come to expect Uncle McDreadful to fuck our shit up. There’s absolutely no surprise in it, no tension. We know and expect this, so if there’s ever some big reveal, we as readers sort of shrug our shoulders and say, “Of course. What else could have happened?”
How you can fix it: Face-heel turns are common with this trope. That cool uncle you’ve had for 180 pages of the book? He’s definitely the bad guy. Heel-face turns, on the other hand, come up much less frequently. Because this trope is so ingrained in media, we expect evil uncles to stay evil uncles. Having it turn out that the uncle has really been trying to help all along (and might just happen to be an ass) or having the uncle choose to help in the end would definitely aid you in making this trope anew. Also, uncles who are evil for reasons outside the realm of envy and greed are certainly less common than those craving succession and power.
Bottom Line: Make this trope your own by changing up character motivation and subverting expectations.