|Theories may be removed if ...|
Many fans note that the descriptions of Jeyne from Catelyn Tully's POV where she is described as having good hips, whereas Jaime Lannister's POV describes her as having narrow hips. This led fans to believe that the real Jeyne Westerling was replaced by an impostor. When Jaime meets with Sybell Spicer, Jeyne is the only daughter present, leading some to believe that her younger sister Eleyna Westerling was substituted for her. It is speculated that the real Jeyne may have remained at Riverrun pretending to be a low-born maid, or that she may have escaped with Brynden Tully, the Blackfish.
- While Eleyna isn't explicitly mentioned in Jaime Lannister's POV, her absence isn't noted either, despite three of Lady Sybell's children being expected to be there. Also, Eleyna is only twelve at the time, and shortly before Red Wedding was still described as a little girl rather than a maid. The girl Jaime met at Riverrun looked to be fifteen or sixteen. Elio García and Linda Antonsson of Westeros.org believe the discrepancy is a mistake.
Current editions of the book remove a reference to Jeyne's hips entirely, suggesting it was a mistake. 
One of the reasons for her possible switch may be her carrying Robb Stark's offspring. This is supported by the reveal that she was taking fertility herbs before her parting from Robb. Additionally, Jaime Lannister reveals that as part of the Westerling's influence in the Red Wedding, Jeyne was promised a good marriage, so they would have no reason to switch her.
- For this to be true her whole family would have been involved, i.e. her father, mother and uncle. For her mother's part, it seems unlikely considering her "fertility potions" were the contrary and she betrayed Robb, this seems a bit farfetched, but it is something that is discussed often on the forums. The A World of Ice and Fire companion application states that Sybell Spicer's posset prevented Jeyne from conceiving.
- However, Sybell may have betrayed Robb to the Lannisters, but it is possible that upon learning that Jeyne was pregnant, feared for her daughter's life and that of her unborn grandchild, organized the switch. Additionally, if Brynden Tully took the pregnant Jeyne without the permission of Sybell and her family, she may have lied to Jaime to ensure they did not receive the wrath of the Lannisters.
- Alternatively, the Blackfish might have escaped with a childless Jeyne on his own. Sybell is forced to cover for her daughter. The other daughter, Eleyna, had been beaten by her mother, allegedly for refusing to hand over Robb's crown. If instead she assisted in her sister's escape or refused to participate in the cover-up, that might explain her bruises as well. Also, Robb had a very Tully look. If Jeyne bears the Blackfish's child, they could easily pass it off as Robb's heir. (With the Starks seemingly exterminated and Edmure and his Frey offspring deposed and imprisoned, Brynden would father the child out of a sense of duty to his late king's memory and his own ruined House rather than any desire for Jeyne.)
The exact nature of the "service" Sybell Westerling did for the Lannisters is still unclear. That she set up her daughter with Robb so that his marriage alliance with the Freys would be broken and the Freys would have a reason to ally with the Lannisters seems likely. As Sybell's grandmother "Maggy" was a witch, Sybell demonstrated knowledge of herbs by making contraceptives for Jeyne, and "love" potions were among the things that citizens of Lannisport went to Maggy for according to Kevan's recollection, it is plausible that Sybell knew how to make a love potion and saw to it that Robb was slipped one while Jeyne was tending to his wounds at the Crag.
Context in the Game of Thrones television show
In the television adaptation of A Song of Ice and Fire, Jeyne's character is replaced by that of Talisa Maegyr in a similar role as the wife of King Robb Stark. It should be noted that in the show Robb's wife was present with him at the Red Wedding and both she and her unborn child were murdered at that time, in contrast to her depiction in A Storm of Swords where Robb's wife avoids that fate as she remained in Riverrun.