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How can I deal with a sister who is extremely pessimistic about everything?

So We have to travel abroad together in 4 weeks, and we started the procedures for it, my sister has started to nag frequently and drags me into her pessimism and negativity (she has always been like this but now it has gone onto an intolerable extent). She has been, until recently, nagging how she won't get the necessary documents although I've arranged everything, after I let her know that every document is being couriered to our address successfully she started to nag how the courier would get misplaced and she won't receive it and her nagging involves creating a whole negative scenario at home like she just turns the situation so negative (I'd compare that with the description about dementors in the harry potter series), and whenever I try to explain that the address is right she would just say, what if they'd deliver it to someone? What if it'd be destroyed on the way? She'd blame me for not apparently "thinking" about every scenario whenever I try to tell her that she'd receive it. Today she received the documents and has found another reason to be negative, now she apparently thinks her visa will get rejected because there's a small correction in one of the documents (I confirmed it with many that it doesn't matter, infact i even contacted the visa office through email) and I told her that, but she doesn't believe me, she's on a constant negative cycle now and she's trying to convince all of us that her visa would get rejected. Last week when I was checking flight schedules, she just randomly started to tell me how our flight might crash? etc

All this constant negativity has started to take a toll on me, I feel so sick, anxious, and depressed whenever I think about travelling with her, I'm generally an optimistic person, but now I'm not feeling the same, I'm constantly in stress due to the negativity that she has and the negative scenarios that she creates at home. Is there any way to deal with pessimistic people?

How can I deal with a sister who is extremely pessimistic about everything?
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