2. How I Met Your 'Mother'
The premise to the series is that Ted, now in the year 2030, recounts to his son and daughter the events that led to him meeting their mother. Note mother, not wife. I may be clutching at straws here, but I think there's some significance in that. As bleak as it sounds, I have a theory that the mother passed away shortly after having her second child. Therefore, the children never got to know their mother. Many years later, Ted's children quiz him on their mother, thus leading into the series. Obviously, because the mother was only just cast, she hasn't been present in any of the scenes between future Ted and his children. Enter Robin. Regardless of whether Robin and Barney get married or not, Robin and Ted will eventually end up together. She will marry Ted and the children will know she's not their biological mother, hence 'Aunt Robin'. It's a plausible theory. Well I think so anyway...