Two of them, actually (two branches of the family, one far north, one near Edinburgh).
This is the one that needs money, for obvious reasons:
https://en.wikipedia.org/wiki/Castle_Sinclair_Girnigoe
It looks like that largely because the Campbells took it over at one point, so the Sinclairs retreated to the woods AND SHELLED THEIR OWN CASTLE. Violently self-destructive for purposes of vengeance? That's my fam.
The other one isn't in much better shape, but while that branch of the family was kind enough to give us a walkthrough they weren't otherwise part of the Gathering and I don't think they took in any of the haul.
https://en.wikipedia.org/wiki/Roslin_Castle It's also associated with Rosslyn Chapel, yes, that one:
https://en.wikipedia.org/wiki/Rosslyn_Chapel Conspiracy theories ahoy!
Oh, and then there's the Castle of Mey,
https://en.wikipedia.org/wiki/Castle_of_Mey , but the Queen Mother took it over as a private residence in the 50s so I imagine there's no need for the clan to raise money for it.
The Sinclairs are really a fascinating clan, but I try not to get tied too much into it as an identity, as there are probably millions and my family is far removed from Scotland.
Oh, as for organization, my mom's the one in touch. There's a website:
https://www.clansinclairusa.org/home.html (see? the tartan's LOUD), email mailing list, and monthly paper newsletter. And of course they have booths at every Highland Games in the US.