WIBTAH if I won't help my sister clean her apartment unless she gives me her cat?

My sister is moving soon, and she wants me to help clean her current apartment. It's an absolute shitshow. Picture 50% hoarder house. I wouldn't mind helping, though, if it wasn't because of her cat.

She had 2 cats, but one recently passed away due to respiratory issues. He had developed asthma, and I strongly suspect his lungs gave up from the health hazard he was forced to live in.

I've asked multiple times if I could adopt the cats due to her living situation and major alcohol abuse. Every time she declined, saying she'd off herself if she didn't have them.

3 months ago I visited the cats and took care of them while my sister was staying at the psych ward due to another breakdown. On the 2nd day, I found the cat. He had died in filth under the couch. I've never been so devastated and filled with guilt.

My sister was (and still is) in and out of the psych ward, and I regret more than anything that I hadn't just stolen the cats one of the times she was away. I'm sure the cat would still be alive if he lived with me.

I'm not making that mistake again.

WIBTAH if I tell her that she either cleans the entire apartment herself to show that she's capable, or I'll help her but I get to keep her cat?