It appears that her mother married, or was married, to a Muslim. By Muslim law and practice that would make her Muslim by default even though her mother raised her with knowledge of the Gospel and she takes the name of Christ. It is interesting that Islam is the opposite of Chrstianity. You can be born into it or enter it by a simple choice. To get out, you have to die or escape with only your life.
On another note:
It troubles me what the human rights group say about the general conditions. At the end of the article it says that Christians are persecuted in 151 countries while Muslims face persecution in 135. This is a false comparison because it does not represent the facts nor deal with issues of the numbers affected, pervasiveness within a country, types of persecution nor intensity.
I also take issue with the idea of religious freedom because it essential contrary to all religions. Every religion by its very nature suggests an all encompassing worldview and if the teachings are followed by every individual in the world would result in peace with higher powers and one another. The only reason an idea like religious freedom can exist is because people are worshipping something else, like mammon. The Word teaches that we become like the thing that we worship. This is the reason that "Christians" could wage wars against each other and commit atrocities like the pogroms against the Jews. Their real god was their belly or their purse.
We have to be very careful and attentive when we hear about events like this sister in Sudan. Before The Lord, we need to see what evokes pity and concern and compare it to the Scripture and where we see that our reaction is of the flesh, we need to ask God for a change of heart so that He might work in us the Love that never fails because it is not from our selves.