I don’t think you are being petty at all. In fact, I’ve had a similar situation happen to me. I think it helps to look at it differently. You chose those clothes and equipment and had them new. They might not have been her style or what she would have chose. You received some extra value in that. You had them new and when they were at their best. Maybe by selling your stuff she can purchase something new she really wanted and couldn’t justify the price of. You gave her that opportunity. That is a gift from you.
I give away a lot of clothes to friends and other families and always preface it by saying, “it’s your now. I hope you get some benefit out of it one way or another. Sell it, give it to a friend, whatever. It’s yours.”
Would you feel better about this if you liked your SIL more? Something to think about.
" I object to violence because when it appears to do good, the good is only temporary; the evil it does is permanent." Mahatma Gandhi
"This is the ultimate weakness of violence: It multiplies evil and violence in the universe. It doesn't solve any problems." Martin Luther King, Jr.