It is made clear in the original miniseries (as it is in the novels) that Princess Irulan (played by Julie Cox) is Emperor Shadam's eldest daughter. However, her sister Princess Wensicia (portrayed here by Susan Sarandon) is clearly much older than she is. Susan Sarandon is actually 26 years older than Julie Cox.
It is established that when a person has lived on Dune for a number of years, their eyes turn the distinctive blue color due to their constant ingesting of the spice (which is in the planet's food and even air). Yet Princess Irulan's eyes are normal, despite the fact she has now lived on Dune for over a decade.