How many Yoda's to beat...

ILS wrote:I'm also unaware of why they did that. Can someone post the source that describes their potentially incestuous bathing escapades?

wait a seg.... does the ones of mortis are incestous......  ?

That's what Apocalypse tells us. Not really much. The soon-to-be-Legends-canon novel Supernatural Encounters gives us an explanation though.

Using both left pincers to wave the Jedi after her, the Killik descended the corridor through several archways to another series of reliefs. The first depicted a jungle paradise, with a small clearing in the bottom of a shallow gorge that emptied into a vast swamp. In the center of the clearing was an erupting geyser, and in the vapor cloud above it floated three ghostly figures, so insubstantial it seemed their limbs had not yet finished coalescing. The trio appeared much younger than in the previous panels, but they were still recognizable as the Father, Son, and Daughter from the forest panels.
In the following two scenes, a walled pool had been built to catch the water from the geyser. In one panel, a fiendish-looking beast with the Son’s head stood at the edge of the pool, drinking from it as the shocked faces of the Father and Daughter watched from the edge of the clearing. The next panel showed the Daughter swimming in a different pool, one located inside a grotto. The head on her shoulders was that of a luminous bird, and it was looking back toward the cave’s pillar-flanked entrance with its beak gaping wide in surprise.
Raynar motioned at the two creatures, first the brutish-looking man-beast, then the luminous bird-woman. “They seem to be changing from one form to another,” he said. “Are they the same beings?”
“Do you think the Ones are made of crude matter?” Thuruht replied. “The Ones are beings of the Force. The Ones take any form they desire.”
As Raynar considered this—and whether that meant the Daughter or another figure might be Abeloth—Tekli stepped forward.
She pointed at the pool in the grotto. “Does that remind you of anything?”
“It’s the Pool of Knowledge that Master Skywalker described in his report,” Raynar said.

Supernatural Encounters:

mmmm looks interesting

Might be missing something, but how are those sources in any way suggesting an incestuous relationship?

How do you think the Force Priestesses were made?

The Ones didn't create the Force. It's entirely possible the Priestesses were either manifestations of the Force or are ancient spirits who learned how to retain their identity and took on the responsibility of passing on this knowledge.