Hi @narges, welcome to the online community! We're really glad you reached out for some support.
It sounds like your sister really trusts you and values your relationship if she feels comfortable talking to you about her anxiety, which is a fantastic thing. It can be really tricky to know what to say when someone we care about is struggling, but often just being there to listen and ask them what kind of support they need is a brilliant place to start.
You mentioned that no one else in your family knows about your sister's anxiety at this stage. Has she mentioned why she might not have brought this up with anyone else? Do you know if she is seeing a counsellor or seeking support from any mental health services for her anxiety?
If you think it might be useful, here is one of our articles on helping a friend with anxiety or depression. It has some awesome tips on how you can be there for someone you care about if you aren't 100% sure what to do. Likewise, if you think your sister might find any of these articles useful, feel free to pass on some of the resources from our Anxiety Space to her and she can check them out for herself.
