What should I do when my sister is upset?

My younger sister is feeling really sad tonight. I don't know why. I tried asking her she didn't say anything she just said she was scared. Me and her and her husband and my friend all live together. Her husband let her sleep with me. I asked her if he hurt her or upset her she said it has nothing to do with him. She just wanted to be close to me tonight. She got into bed and snuggled up against me and started crying. What can I do to help her feel better? Why does she want to be so close to me?
What should I do when my sister is upset?
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