Hmm ..
"The Sentinels -Leaders of the Clans?" - No.
Well, the thing is .. Clan influence stretches beyond Tir.Now. Like the Council of Truth, not every clan recognises Simon Silverstone as the leader of the Clans. In fact, probably only a minority think of him as their leader. However, a military dictatorship is not subject to democratic rules, and as long as he remains in Tir, uncontested, he must be seen as the only real leader of the Clans.
Also, as weird as this might seem, yes .. Silverstone is a subject to democracy as well.
He's not really the clan leader unless the clans want to follow him and do so.
Sir Galahad and his knights, also a clan structure, is in control of most of Avalon. There's no thread started about wether he's the leader of the clans.
The only way for the sentinels to rule the clans would be by force. I don't think it is in Silverstone's interest to start a civil war.