has marked an incredibly bittersweet occasion as she made a heartfelt tribute on what would have been her mother's 68th birthday.MORE: Denise Richards' family home after fleeing California fires – photosThe Real Housewives of Beverly Hills star lost her mom, Joni, in November 2007, following a battle with cancer.WATCH: Denise Richards celebrates birthday with sumptuous cakesTo mark her late mother's birthday, Denise posted a beautiful family photo of her and her mom, as well as her sister,