With the reality warping power being the new black for accomplishing seemingly impossible things in the Marvel Universe. I don't think it describes how things are brought about accurately. I find the Scarlet Witches probability and spellbinding capabilities intriguing. So, what if in fact the children she created aren't really from pieces of Mephisto's soul, but a way of making Wonder Man the surrogate daddy of sorts. What if her probability/magic powers were able to steal genetic material from Wonder Man to blend with hers to make new lives and souls. Wonder Man was used as a template to create the Vision, and he and Wanda have real history. He is the perfect candidate to "father" her children in that sense. It also makes the mechanism for the impossible more plausible and well interesting.

Considering all the mystery and unpredictability of her powers, I think the spell to create her children involved some kind of mystical IVF (in vitro fertilization) with Wonder Man as the template. She didn't create life or steal part of Mephisto. Instead she hijacked Wonder Mans ionic radiation sperm and combined it with her eggs in a mystical incantation. It make sense since her husband Vision is modeled after Wonder Man.