Oh my goodness that is so awful sending hugs to you
I had a kitten that died suddenly and I never found out why so I know how frustrating it can be. Have a good look around the house/front room to see if there is anything that she could have picked up, maybe she choked on something.
As for your other cat I would dish out loads of cuddles over the next couple of days.
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RIp Tigsy my little baby girl 21.09.09 to 10.02.10
Proud Mummy for 6 days to gorgeous Poppy (4/09/09 till 10/09/09)
I hope your chasing butterflies in kitty heaven little one