Interestingly Thule Racks has recently purchased a car rack company called SportRack. And SportRack has a couple of bike racks that will install directly to your 1999 Toyota Sienna roof rack cross bars without requiring any kind of adapters. They are the
SportRack ABR622 Nomad fork mount bike rack and the
SportRack ABR611 Nomad upright mount bike rack. Both of these are quickly becoming popular because of their great prices and that they work with most kinds of bikes. The only drawback with the forkmount version is you need to use a cable lock to lock the bike to the rack, most fork mount bike racks use a lock and key that inserts into the skewer that you tighten the bike fork to the bike tray fork block. The upright mount version does come with a lock and key to lock the bike inside the rack.