James Welch, who's KILLING CUSTER, is based on Indian accounts of the battle, has this to say about Sitting Bull's role (pp 157-158).

"Which brings up another mystery surrounding the Battle of the Little Bighorn: Where was Sitting Bull during the fight? Depending on which scholar you read, he was making medicine in his tepee, he was riding at the front of his warriors shouting, 'Brave up, brave up," he was riding herd on the women and children in the center of the camp, or he was cowering behind them. These various accounts of his whereabouts were furnished by Indian people who were in the village. Some of them are honest recollections, but some are inspired by adulation or jealousy...

The truth is that Sitting Bull was forty-two years old at the time of the battle, an advanced age for a warrior. By contrast, Gall was twenty-nine, prime fighting age... And (Sitting Bull) was a chief, not an active war leader.

The most probable sequence of Sitting Bull's movements, based on the majority of accounts, is that he mounted shortly after Reno's attack, shouting encouragement -- definitely not instructions, as some scholars have suggested, since Indians of that time in the heat of battle were not inclined to listen. Then he made his way through the village, gathering women and children and the old ones, persuading them to stay put in the safety of the village, or just west of the village, near the hills...."