Praise be to Allaah.
If your talking to your friend will not result in any negative consequences such as her brother trying to talk to you or contact you and the like, then there is nothing wrong with your talking to her, because that ban was only because of negative consequences that might result from it. But it should be limited only to talking to her, and not visiting her. It is better if it is done with your family’s knowledge; perhaps they know something that you do not. Keeping company with some people may lead to damaging one’s reputation or being thought badly of, and news of that relationship may reach the husband.
May Allaah help us and you to do that which He loves and which pleases Him, and divert fitnah (temptation, trouble), both apparent and hidden, away from us.
And Allaah knows best.