Urwendil and Urwendur morphologically fit neither Quenya nor Sindarin. They’re macaronic constructions from an apparently Sindarin word (Urwen is Lalaith’s true name, not mentioned in the Silmarillion) and Quenya suffixes. Those suffixes lived into Sindarin but went through the proper mutations of that language. Hence Ëarendil > Ëarnil, Ëarendur > Ëarnur.
Thus, in proper Sindarin we would have *urwennil and *urwennur from Quenya *urvendendil and *urvendendur.