brigida was fading.lock was ticking. >> when i saw her and i knew that she was dying, she was in a lot of pain. she was in excruciating pain. and i remember being in the hospital room and i was like, i want to be close to my sister. so she put herself through an insane amount of pain just to turn her body to look at me. >> how close was she to dying? >> i watched my sister almost die, i knew that she was gone. >> a family anguish deepens, and the race to unravel this mystery shifts into overdrive. >> coming up. >> your daughter is getting weaker, and weaker. she was losing her vision? >> she was losing everything. >> for her husband and young son, were they in danger as well? >> i have a child in the equation now, i have a spouse. let's make sure the child is safe, now they have not been exposed. >> when dateline continues. been exposed. >> when dateline continues why hide your skin if dupixent has your moderate-to-severe eczema or atopic dermatitis under control? hide our skin? not us. because dupixent