"You are a modern mother."
Translation:Tu es une mère moderne.
Because you can address a person in two ways: a casual one, with "tu", and a formal one with "vous". You can use the casual form with your friends, your family, people you know well or who are younger than you (children for example). The formal one is aimed at strangers, your boss, your colleagues, etc. :)
Although in BrE, "mum" is not considered to be informal, unlike "mom" in AmE.
"Vous etes une mere moderne" makes no sense. If you use the article "une" the "you" can only be singular "tu", a modern mother, no?