Are Paris Hilton and Doug Reinhardt secretly engaged?

The hotel heiress has made no secret about wanting to marry the former Hills regular, but she took it to another level this past weekend while accepting an award for her charity work...

"I'd like to thank my future sister-in-law Casey [Reinhardt]," we're told Ms. Hilton said from the podium at the Hearts for Hope benefit in L.A.

Doug wasn't at the event, but both his sister and mom were, and they were sitting at Hilton's table.

If Doug does propose to Paris, perhaps he can take some pointers from his mom. "At one point, Paris reached for Doug's mom's huge wedding ring," says an eyewitness. "She looked like she was in awe."

And get this, the Reinhardts are such big fans of Paris, they'll even pay to hang out with her! "Paris auctioned off a party picnic at her house for charity, and the bidding went all the way up to $6,000," says our source. "Even though Casey can obviously party at Paris' house anytime for free, she went ahead and took the final bid."

In May, Paris gushed to our Hollywood Party Girl, "[Doug's] gonna be my future husband!"

Sounds like they could be picking out china patterns any day now.