So this is kind of a stupid theory but might as well share. So I noticed that, when in a doused state, Flame Princess has very similar colors to Princess Bubblegum. You can also see that most of the members of the fire kingdom are sentient fire beings or molten lava rocks or peppers and such, but FP is more of a maroon girl with flame elemental type abillities and attributes. So, if we go back to the episode, "his hero", in Billy's theme song, it says that he beat the "fire count" (originally titled the "fire king", perhaps the flame king's brother, who he "snuffed out" meaning maybe he told billy about the fire king) and saved the cotton candy queen (or maybe it was cotton candy princess). Perhaps the men of royalty in the fire kingdom, being evil, had a history of stealing candy people as their brides, like the ice king, because there is a picture in the candy kingdom that suggests the cotton candy queen might be PB's grandmother or something. So perhaps, the flame king stole PB's mom and had Flame Princess, being half candy, half fire elemental. In the promo art for Burning Low, PB is looking at a young FP, trapped in her lamp. She seems sad because she has had her locked up in there for her elemental matrix. Perhaps she is sad because of guilt or because she has just locked up her sister, and perhaps this is how she knew about flame princess and had her locked up. Whatevs. Just my theory. Hope you enjoyed :)