Tenipuri is the main series and the major source of characters. Angel Sanctuary provides the impetus that moves the crack plot forward so you should know a bit about AS (basic knowledge of archangel, demons, etc. will also suffice). Readers should know Tenipuri characters. The in-jokes are funnier if you're familiar with all of the series, but it's hilarious regardless of whether you've seen/read everything. With the exception of Eriol, there's only one piece with Clampverse characters(no specifics, not even names).

I wouldn't call it spoilerific. There are minor spoilers for who the characters are. But nothing more than you would get from the first chapter/episode of a series if you decided to pick it up. You get some events and some subtext, but there's no mention of how events happened. And as an experienced anime fan, you'd be able to spot a sibling complex from a mile away, so no spoilers there. ^_~ Go for it. It's great fun. And read the comments at the bottom of the main page. They're fabulous. *grin*