Sister using my baby shower gift?

My grandmother got me a bumbo seat for me as a baby shower gift and gave it to my sister to keep it at her house until my baby shower. Now my sister has been using it for baby sitting her niece. Normally it wouldn't bother me but my mom also said she saw one she wanted to get me. My sister said if she did she'd keep the one my grandmother got me. Not asking but saying she would. She sees her niece once a week for 2 hours and has no kids. Am l stupid for letting this bother me? I just don't think it's right to use something without asking or saying you'd keep it if I got another one. She's been stocking up on her own baby stuff even though she said she's not having kids for 4 years. I also understand it's at her house but I know most people wouldn't do that.

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