Call it artistic license, the TV show is loosely based on actual history and is meant for entertainment only.
After some further investigation Rollo the 1st married Poppa Duchess of Normandy as his 1st wife, hence he claimed the title of 1st Duke of Normandy and established the line of Normans that included his son with Poppa, William Longsword and later William the 1st, The Conqueror and a long line of British Royalty. They were my 31st gg grandparents. His 2nd wife Gisela (if indeed she really existed, as there is still controversy surrounding her) may have been the daughter of Charles "The Simple" grandson of Charlemagne. Charles the ruler of Paris is I believe the person depicted in the TV series Vikings. There is no record of any issue from Rollo and Gisela. Further there is no record of Rollo and Ragnar being brothers as the actual existence of Ragnar is considered a possible myth whereas by comparison there is much more documentation of the life of Rollo.
