Pippa Middleton was all smiles while stepping out in London on Thursday, and it's pretty obvious why: the Duchess of Cambridge's younger sister got engaged to her longtime boyfriend, James Matthews, earlier this week after the financier popped the question during a stroll through the Lake District. We first got a glimpse of Pippa's gorgeous new ring when she left her home on Tuesday, and despite the diamond's size, somehow Pippa is still walking on air. The only snag? Pippa's sister, Kate, may opt out as a member of her wedding party — People reports that it's a "tricky situation" and that Kate wouldn't want to "upstage her sister" by being her royal maid of honour. Veteran royals author Judy Wade added, "Kate would want to be in the background as much as possible."