Here's something I've been thinking of while I was unloading the dishwasher. It'd have to go over the theory that Higurashi and Umineko are in the same universe, but maybe this will probably help out with other theories and what-not.
Spoiler for Bernkastel = Very lengthy.:
Alright, let's think of Yasu's situation here. Her life sucks. Plain and simple. As she was working she only had Shannon (who is rumored to be a complete imaginary friend) Then one day Yasu discovers things about magic and witches (probably because Kinzo's a fanatic, and she must've come across some occult books while cleaning the library) and she is fascinated by the things of witches. She imagines Gaaptrice tormenting her and what-note. (Me and my sister Lulu do this sometimes. We imagine anime characters "hanging" out with us. I know it's crazy, but it helps with inside jokes between me and my sister. We like to roleplay alot.) Anyway, Yasu's life continually goes down the drain I guess, then decides, "I want to be a witch!!!1!" and "becomes" a witch. In other words, she imagines herself as one. She runs around unlocking doors and windows to prank the servants and make them think a witch is about.
Now think a little about how so far I think of how witches come to be. Beatrice the Witch came from Yasu's imagination, Virgilia is only based off of Kumasawa so she's still "imaginary". As for every other witch they're "birth" is completely unknown, so we have no evidence whether their imaginary or not.
Okay, now how about an Umineko Rika? A Rika who is not the Higurashi Rika but is really based off her. Just a coincidence they're almost the same. Yasu and Rika are almost the same age when Yasu starts working as a maid. Now I KNOW Yasu is the only maid with that age (She thinks she's 6, but is really 9, so..) so Rika didn't go there. Suppose Yasu got the idea of "imagining" a witch from a different girl? What if a girl she knew from Fukuin went:
Girl: I'm a witch named Bernkastel! Nipah!
Yasu: ._. *walks away slowly*
When Yasu began to "be a witch", she decided to create a form for Bernkastel, because the girl who "already was a witch" inspired her from the beginning.
I know this is extremely stupid, but I am just thinking out loud here. And also maybe Yasu just read the novel "Higurashi" and based a witch after the character Rika. Simple as that.
Lol, really stupid theory above. Be very afraid. xD