You are going to need to consult a probate law attorney to see if you have any legal recourse against the trustee who made this distribution. I would consult one who does litigation versus one who does estate planning. I don't know if that vague descriptiion if "good enough" or not. This one tends to be very state specific.
If you wish to pursue this one, you probably will have to file some kind of law suit against the trustee. What you may need to ask yourself is whether the costs of litigation (which could easily be thousands of dollars), will outweigh your late uncle's actual financial interest in this property and whether your legal fees and costs could be recovered in any law suit, if successful.
Also, merely winning a law suit is only the first step. You also get the joy of collecting on your judgment, if you get one in your favor. If the trustee is a "deadbeat" collection could be difficult.
It is the trustee's problem to get back the money from the other 19 folks involved, if it comes to that.
If you late Uncle's financial interest is, for example, only $500 or even $5000, I would forget about it. If i was $500,000, then you obviously have something that might be worth pursuing at least to the point of getting a legal consultation.