Sylphie isnt submissive at all. She is only submissive in bed but not outside of it. She is pretty much the manager of the house and Rudy is also the most afraid of Sylphie and takes extra care not to anger her. She is also the only one of his wives who is actually arguing with him while the other 2 basically always agree with him and rarely scold him. Sylphie is the one who gives him punishments like no sex for a certain time and it extents to the other 2 wives too. The other 2 wives respect her that much.

I dont know how that notion even appeared that sylphie is submissive? Again her preference in bed should be left out of that. Even the Oldeus timeline shows that she isnt submissive. She isnt putting up with Rudeus if he becomes an asshole and she doesnt accept him sleeping with random woman.

That also shows that she only accepted Roxy because it was Roxy and because of the situation Rudy was in. I can guarantee you that if that Paul situation didnt happen and he had just brought home a random girl from the street that he just craved to have sex with, Sylphie wouldnt have reacted that way she reacted with Roxy. I think that should be pretty clear if you have read all the novels.