I remember one day thinking through things she enjoyed despite being old and arthritic: lolling in the sun, sitting on my lap, snuggling along my armpit at night, being petted. She had stopped doing all those things, and no longer sought contact or responded with stretchy paws and bliss face when I tried to cuddle or stroke her. There was no dramatic downturn in her health. She just gently got skinnier and and skinnier and slept most of the time. At some point I realized I couldn't remember the last time she'd purred. She was just existing. She wasn't actually enjoying anything anymore.
Most wrenching decision of my life. This was over eleven years ago and damn if I'm not welling up over it again.
The only comfort in any of it and it was small comfort at best was knowing that I'd upheld the promise I'd made by adopting her: to not only give her a good life, but also to give her a good death. I didn't wait too long. She didn't have to be in blatant, horrifying pain or distress for me to get the message. (The downside is sometimes I wonder if I couldn't have waited just a bit longer, had her for just a bit more time. But I'm willing to live with that question because the alternative waiting too long is too awful to think about.)
The vet came to the house, and she got to die on my lap, in among her familiar smells and touches and sounds.
The night I ended up deciding ... I'd crawled into bed, crying, and she hobbled her old self up to my face and made a chirp. I said something like, I don't know how to do this, I can't do this, can't you give me some kind of sign if this is what I'm supposed to do? And she nudged herself under the blanket, leaned her knobby spine against my chest, and purred.
My heart goes out to your mum. I'm glad you're going to be there with her. My mom-type was with me, and I was grateful for that, even though my real grief didn't hit until after she'd left, when I was alone. Hopefully your vet will be able to let your mum hug the bodies afterwards for a while if she wants to. That was important to me -- being able to really hug her without feeling like I was hurting her, and also, I really needed to know she was gone.
Shoot, I wrote a book. Sorry!