(SCIENCE!) Remilia does not typically eat the head or brains of her prey. The remnants become vargouilles, which resemble the vampire what spawned them. In short, they become little flying Remilia heads, with longer-lived ones growing itty bodies, and the longest budding off, through a terrifying process, tiny tiny Remilia-vargouilles.
Remirya don't seem to have any sex drive, honestly. Any birthing they do has ever been depicted as being raped (by Alices, as mentioned in one of the translated stories) or perhaps there are some consentual, to some extent. Some Sakuya-types probably get a little ... too excited for their own good as well.
I like the idea about vargouilles because it does explain how some Remirya are depicted with "baby-sized Remirya" - and calling them its "babies." All other evidence says they give birth by spewing something out of their chin. There seems no possible way a full-bodied Remirya could come out of that orifice, and any Yukkuremi that reach a body-stage have to be too old to be consider "babies" anymore.