Louis Lepine had 3 sisters, Marie, Anne and Caroline and 1 brother, Raphael (aka Jacques).His ancestry is reasonably well documented in his biography by Jacques Porot.There is no mention of a brother named William.
The name William Lepine was in England several centuries before the 1800's.The Lepine families in Canterbury and London were Huguenots who fled to England from France.The "earlier" William Lepine's were still using Guillaume (French for William) into the early 1700's.
You haven't said why you think he was related to the famous Louis or why you believe he emigrated from France to England.If it's undocumented "family legend", you could possibly find yourself going down the wrong road.I know from experience - that's why I have a copy of Louis Lepine's biography. :-)My mother (a London Lepine) always said that Louis was related to her family.Perhaps he was, but since my Lepine ancestors have been entrenched in London since the late 1600's and Louis' forebears were from France and Spain, it's a bit of a stretch. If you think he may be a native born Englishman, I'd be pleased to see if I can help.