It's called taqiyah

I've never heard of this so I'm looking into it now and it doesn't seem to match exactly what you're saying, although the ideas are similar. According to what I'm reading, it states, "There are several forms of lying to non-believers that are permitted under certain circumstances, the best known being taqiyya (the Shia name). These circumstances are typically those that advance the cause of Islam - in some cases by gaining the trust of non-believers in order to draw out their vulnerability and defeat them."

Based on this, it would seem that lying to their families would not be covered under taqiyya because their families are also Muslim believers (maybe there's another word that would match my example a little better). The girls I knew lied because they did not want their families to know that their actions were not moral according to Islam (the girls were not religious but their families were, so they lied to make it seem like they were good little Muslims who didn't drink alcohol and have sex before marriage when in fact they always did).