Oh, {{{{{Always}}}}}} I'm sitting here crying knowing the heartache you're feeling. Two of my furbabies crossed the Rainbow Bridge recently - My precious friend who went through all of life's trials with me (thus far) - my 20-year-old Morris. I had to take him to be put to sleep and that was so very hard.
When Morris was 14, I adopted Kali who was 3 at the time so I wouldn't be "petless" when I lost Morris. Six months before Morris died, similar to your Sandy, I found Kali curled up on the couch having crossed over the bridge. She never showed any signs of illness or distress.
Sandy has quite a welcoming committe awaiting - Morris (who was such a little gentleman) is so sweet. Kali was a little more...ummmm...spirited, but she loves to romp with other cats.
Have you been to the Rainbow Bridge website chatroom? I spent about a week in there when I lost Morris.
If you need to talk, please PM me. I love animals!!