I've read up on when and what the signs of a lonely rat are.
Apparently it may take Peach a couple of days to realise her sister isn't coming back.
If it hits her hard she may end up hurting herself in a worst case scenario like chewing through her own tail.
I'm so scared that it may happen and contact with me won't be enough.
I think these next few days will be the most important to her that you're there for her, as she's coming to terms with this change. What you've been doing sounds like the best way to offer closure for her, too. Hopefully she doesn't respond to the extremes that you're reading about. But if she does respond poorly, it's not your fault. You're doing everything, and I mean that. Like C said, we'll be here if somethig happens. Hopefully it doesn't.
I've mentioned it in the shoutbox, and I feel weird bringing it up with the timing, but I do have experience with a rodent dying of a broken heart so I completely understand how devastating of a turn that can be. I really, really hope Peach copes alright. For both of your peace of mind. <3