If the other siblings can get an order to get him off that property, that is a court system. If not, they have lost any chance of an inheritance.
OK, so now we come to how law is enforced without force. I've said this before, elsewhere, but I'll say it again here.
So, the two siblings who just want a share of the property agree to go to arbitration to settle the dispute, but the a-hole son doesn't agree, he believes that as the first son, he is entitled to the whole thing. He hasn't harmed anyone, unless the other siblings had residence on the property and he won't allow them to stay, but we'll assume he moved in after the father's death and the other siblings likewise had other dwellings. But he still refuses arbitration to resolve the dispute. No harm means no justification in kicking him off by force, so that is off the table.
However, there is still an option available to the siblings. They can make it known to all and sundry that the offending son is refusing arbitration. Since arbitration is the means by which disputes are resolved, anyone who sees that knows that should they get into a dispute with him, it's is not likely that he will accept arbitration on that, either. That means that they are not likely to deal with him. He can't force others to do business with him, so if they decide not to, he's out of luck.
You might think (and maybe he does, too), "Oh, no big deal," but imagine: He can't get clothing, because the clothier knows that if there's a dispute, he won't go to arbitration. He can't get food, because the grocer knows that if there's a dispute, he won't go to arbitration. He can't get any of the services we take for granted in modern life, because the proprietors know that if there's a dispute, he won't go to arbitration. Most importantly, he can't get protection, because the protection companies know that if there's a dispute, he won't go to arbitration. He has, by his own actions, made himself literally an outlaw. He's on his own.
That's a pretty mighty incentive to go to arbitration, yeah?