If the surname already ends in an 's' (e.g. Venus Williams) then you simply write it with an 's'. If it did not end with an 's' (for arguments sake, say the name is 'Venus William') then you can add an 's' if you are talking about the Williams i.e. you are referring to more than one member of that family by surname - as you have seem to have done when formulating your question. Does that help?
Oh...one more thing. If the family surname ends with an 's' - let's stick with 'Williams' - and you want to refer to them collectively, I believe you would say 'the Williamses'. But, personally, I would say the 'Williams family' or the 'Williams sisters', rather than 'the Williamses'.