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While I understand you wanting to remain on good terms with your sister, I don't think it warrants putting yourself and your home at risk since she has done things in the past. I would let her know you love and care for her but are not comfortable sharing a key. Yes, she might have a big reaction, but if you give her the key, it sounds like she won't care for you or your things in the way you desire.
It is possible to have an open discussion without it being confrontational, at least from your end, but you need to be comfortable. You can wait until she raises it again and let her know that you have given it some thought and you just are not comfortable and hope she can understand your feelings and your boundary around it.
Let me know any additional thoughts.