Am I being unreasonable?

My sister texted me saying her laptop died and our mom told her to ask me if she could borrow mine. She is a freshman in high school, my laptop is from when I was a freshman in highschool 7 years ago. Hers is only 2-3 years old so I don't know what she did to kill it. Mine is on its last leg and has to be plugged in at all times incase it decides to charge but I said ok anyways. I made a guest account so she could use power point and do her project. Then she says last minute she has to take it to school everyday for a GROUP project that others aren't doing their share on but the teachers don't care. I said hell no you're not taking MY laptop to school with a bunch of kids. Now both my sister and mom are getting mad at me because I don't want MY personal belongings that I paid for (custom built) at her school.

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