How would you deal with this?

We are temporarily having my sister in law live with us, along with her 3 year old. Her 3 year old and our 4 year old are best friends and generally play so well together but often find mischief.

Today I was giving my 6 month old her bottle when my niece came and told me there was a water mess. I went up to my daughters room to find they had filled up a bin with water, along with a toy chest (full of toys), and there was also water all over the floor plus spilled milk lol. This honestly happened in about 15 min. Please no judgement on why were they playing unsupervised. I usually check in on them frequently but was having a busy baby moment.

Anyway we cleaned up the mess and a number of toys were ruined and had to be thrown out. My daughter loves water and is frequently reminded no water play without mommy or an adult. I was frustrated by the mess and my husband and I agreed she’d be sent to bed once clean up was done (it was about 30 min before normal bedtime). She was really upset that she couldn’t continue playing but we felt she needed a serious consequence. Meanwhile my sister in law gave zero consequences to my niece and instead gave her a snack.

I know it’s not my child and she can choose her own consequences but how will my daughter feel knowing she’s the only one that got in trouble? I don’t know how to explain to her why only she got in trouble (plus her toys got ruined through their actions) and her cousin got a reward.