WIBTA to not return a pair of black leggings to my former employer?

I used to work at a really cool place, and part of working there was wearing a super awesome uniform; without spoiling too much of the company, it's heavily themed by medieval times.

I had a cool red tunic and black leggings, and a fun hat; I miss that job. . .

Unfortunately, it was 50 minutes away, and some days I'd literally lose money because there was only one show, and gas was too expensive to drive all the way there for only a couple hours to then drive all the way back . . So I had to quit.

Fast forward a few years to a couple weeks ago, I was going through my closet \[moved a couple times, so I had boxes with random stuff piled in\] and I found that I accidentally kept one of the laundry bags. It didn't have any of the embroidered tunics or hats or sashes (those I would have returned ASAP), but the bag had one pair of black leggings. They're just ordinary black leggings, no logos or anything.

It's been years since I left that job, and no one has contacted me at all regarding a lost pair of black leggings.. or the kinda gross laundry bag. . .

So I decided to add the leggings to my wardrobe, and toss the gross laundry bag. These leggings complete my outfits pretty well, so I've started wearing them fairly often over the last couple weeks. My sister asked me where I got them and I told her that I accidentally kept them from my old job and that I didn't really think they were important enough to return; she told me that I could get into serious trouble if they catch me wearing them in public. . . and I just kinda acknowledged her concern, but also dismissed it a little. . . I mean I get it, it's technically their property, but also they are cheap ordinary black leggings, and it's been multiple years since I left. . . and even if someone from the company saw me wearing ordinary black leggings in public, which already is very unlikely to happen, it's not like they can determine that these are somehow their leggings. . . They're the same you'd get at a Walmart.

My sister said she was disappointed in me, and that she doesn't like that I'm being dishonest with a company that treated me well and that I still love, but like I don't really see it as being dishonest. Had I found this bag years ago when I had just left, and decided not to return it, then I could see how that's kinda bad apples, but it's been way too long, returning a pair of cheap leggings does not matter and doesn't say anything about my credibility in my opinion.

My sister and I were supposed to go to lunch together today, but she cancelled when she found out that I had not returned the leggings yet. . . I said, "it's not that serious, please forget about the leggings" and she said, "not until you return them."

I really feel like it's not that big of a deal though, AITA?