Slick, my heart breaks for you. I had to make that choice for my Grandma kitty in November. It's never easy even when you know it's the right thing. I'm sure Grandma will happily show her around the Bridge along with all our dear pets plus Gini and Phred. She will leave your arms and step into theirs. You gave her so much love in the last years of her life and got her out of that shelter. She won't forget that or how much she loves you.
Prayers for you both.