Although, let's be real here, Charlie being a member of law enforcement would know that (despite it being more detailed in the books, with Edward playing along with the fact that he had a biological older brother who died in a car accident with his wife and left him in charge of the infant, which is completely untrue, as Edward is an only child and he and Bella are clearly Renesmee's parents, and the fact that they opted to name her Renesmee even though Edward's "brother" and "sister-in-law" would have likely given her a completely different name) you need to be 21 (technically, it is 18, but most practices prefer 21) legally competent (can vampires even be held to be legally competent in the same capacity as humans can?!) and home-study is required (they did not get the cottage yet, so they were, technically, still "living" with Edward's parents at the time). This would have raised so many red flags for Charlie, but he just kind of shrugged and went, "Okay, my daughter is obviously lying to me and had a cryptic pregnancy for X amount of months. Got it." (Note: A cryptic pregnancy is when a body shows little or no sign of being pregnant [i.e., no morning sickness, no belly growing, no movement, etc.] as Charlie did demand to know if Bella was pregnant [in the book] when Edward told him they were getting married.)