I babysat my nephew and my SIL came home PISSED. I genuinely don't know why??

So this happened a few days ago and I genuinely don't know if I did something wrong.

Okay so quick context: I'm a college student, I have a much older brother, and his kid (my nephew, kindergarten age) is usually watched by our mom after school until my brother and his wife get off work. But a few days ago my mom had something come up, so I got drafted in as emergency babysitter.

First time alone with this kid. Just the two of us. A little awkward but I was trying my best, we were playing and stuff, and then he goes "can we go get a snack?"

His absolute favorite thing is this cotton candy sold at convenience stores. You know the one. It comes in a little transparent cup. Super cute. Apparently the parents + my mom have a whole system where they buy it every other day, and when I called my mom she confirmed today WAS the cotton candy day. So off we went.

Sold out.

We hit up a few other stores. Nothing. One place said they don't even carry it. I told him to pick something else and he just... looked at me with this tiny disappointed face and said it's okay, let's just go home.

I'm DEAD. This child was so brave about it I couldn't handle it.

On the walk back I randomly remembered that when I was little we had this home cotton candy machine, like a little toy one. Checked Danggeun¹ on a whim and someone in the next apartment complex was selling a brand new one cheap. I bought it. We went home and made cotton candy together and he was SO happy, running around, genuinely losing his mind with excitement.

And then my SIL came home from work.

She looked at me like I had personally wronged her entire family line. Ice cold. Said she'd appreciate it if I never did this again. Then started packing up the kid's stuff and left before I could even ask what happened.

I've been going through every possible reason since:

1. She's grossed out that I bought it secondhand? (It was brand new, unopened. I told her this.)

2. She thinks he ate too much cotton candy? (It's a tiny slow home machine, he ate less than his usual cup amount. Also told her this.)

3. She's worried he'll beg to use the machine constantly? (I actually thought of this beforehand lol, I told him we were just borrowing it and had to return it. My plan was to ask my brother and SIL what to do with it: keep it at my mom's, give it to them, whatever.)

I covered every base I could think of. I even PREEMPTIVELY covered base #3.

Parents of Reddit, what did I actually do wrong here?? I was genuinely just trying to make the kid happy and now I feel like crap about it :((