If I authenticate a second iTunes can they browse and watch movies I've bought?
Yes. You can authenticate as many as 5 iTunes computers that you own or control according the the terms of the iTunes store.

There should be a download icon that displays in iTunes when you have purchased some new content that he can download. This process is designed so that a single user with multiple machines can easily download content on all of them (think iMac, Macbook Air, etc.). 

Since you mentioned cloud, you will need to subscribe to iTunes Match for this computer to have streaming access / download access to movies that were sold from iTunes. Movies and videos you have in your local library do not get uploaded to the cloud, so you would need to get those to your sibling's Mac or set up a network where you could use Home Sharing or another service to stream content that's not on Apple's iCloud/iTunes CDN.