Nicole Richie better start trying on bridesmaid's dresses. Her BF, Joel Madden's twin brother Benji is getting hitched! Benji popped the question to GF Sophie Monk on Christmas Eve, E! News reports. It's been a pretty quick ride for the couple -- they were first seen together in public in September. "They are incredibly happy," Benji's rep says. And Sophie, who's an Austrailian singer/actress, gets little bro's seal of approval. "I love [Benji's] girlfriend," Joel told People in October. "She's so cool. I hang out with her; we have a great time. She's really similar to me -- she's very care-free." Joel's only five minutes younger than Benji -- could his and Nicole's engagement be far behind?