What is the right thing to do?? involves my sister and a plant?

Give her the plant back, not because she deserves it, but because she is your sister and it would make her happy. You have your own so it can't really be that big of a deal giving it back to her, and that way-there will be no hard feelings-instead she is likely to be grateful to you and impressed by your attitude instead of feeling regret that she gave the plant to you and she missed out.