Was Mariette a bit of a slut?
In the very first episode, they all think Mariette is pregnant - and apparently not even Mariette has any idea who the father might be. And the entire family is extremely blaze about it. Yet, recognising the need for the baby to have a father, they try to set up the match with Charlie as quickly as possible, so that he might think the child is his.
In the end it turned out she wasn't pregnant after all, but she must have had several careless flings going on at the same time.