Why would the beginning of the article claim them immortal when 3 out of the 7 sisters are dead ?
Alassra faded from existence restoring Elminster (admittedly she didn't "die").
Sylune was killed by a red.
Qilue was killed while playing host to Eilistraee's avatar.
The background is that they were Chosens of Mystra. It stopped their clock regarding lifespan, but they could be killed. Quite similar to undeads' immortality, left alone they could go on forever, but in the face of enough violence they crumbled down.
