Depressed sister

ReecieJ
I've had a really weird feeling lately (it usually happens when something's going on with my twin sister), so i called her and it went straight to voicemail. It was weird because she texted me a few hours before, so i drove over to her house (she's 45mins - an hour away). We have keys to each other's houses and when i knocked and rang the doorbell she didnt answer. Im freaking out like anything and i let myself in, to see her sitting on the floor crying her eyes out. I managed to calm her down and resting, and called my husband to let him know that im spending the night with her. She was hysterical and begged me not to tell our parents. I have NO idea where to go from here; shes sleeping now and im literally afraid to leave her alone.