At the beginning of the episode, Laura explains in her narration that the past December marked Rose's birthday and the news that Laura was pregnant. It doesn't take an eagle eye to realize what's wrong with this. First of all, Rose was absolutely not born during the winter, as evidenced in the summer-like weather that was going on during her birth episode, Season 8's Days of Sunshine, Days of Shadow. Secondly, if Rose really did turn one at the same time Laura found out she was pregnant again, then she would have been almost two years old when her little brother was born--and yet throughout the episode, it's obvious that she isn't even a year old yet--probably only about 6 months, which correlates fairly closely with the amount of time that passed since her birth in Season 8, but it doesn't make sense with what the current plot is telling us. It's an absolute joke to say that Rose and her biological baby brother are only months apart. Physically impossible! My only question is, WHAT were the writers thinking???? The only logical explanation is that Michael Landon couldn't find twin toddlers of the right age to play Rose at this time, so their only choice was to have her remain a baby.
Note: Actually, the real Rose Wilder was born on December 5, so what the narration said about that was at least true.