I'm so very sorry, for what you had to go through. I think you did an incredible thing by adopting her, caring and loving her as you did, and gave her the final gift of letting her go peacefully, the most difficult part of pet ownership is the final farewell. I hope you take comfort that Tsuki had a great life while she was with you and Sera, and had a very happy, content kitty life. She is in peace now.
Sending you a big warm hug for the bravery and courage you had, and wish you peaceful moments during this dreadful time. My heartfelt condolences to you and your household. Pets to Sera, too.