There is a good technique called 'broken record' you effectively acknowledge/ even agree with what they are saying and then politely say no and repeat as much as needed. Never giving a further explanation. So in this situation an example would be:
Her - 'Hi everyone, you have to try some of this cake I made it especially for you all, its a new recipe'
You: 'wow, yes the cake looks amazing, how lovely of you - I am avoiding sugar so won't have any but thank you.
Her 'just a small piece then, it won't kill you'
You 'I am sure a small piece wouldn't kill me, I am avoiding sugar so won't have any thank you'
Her 'even a tiny piece just to taste'
'Even a tiny piece, not for me thank you'
Her 'don't be so boring, look everyone else is having some'
You: 'I know I am boring aren't I, I can see everyone is loving the cake, but not for me thank you'
Her 'After all the effort I have gone to you don't event want to try it'
You 'You have gone to so much effort, so lovely of you - I am avoiding sugar so won't have any thank you.
It works as you are effectively agreeing with them but still politely refusing and not changing your reason.
If she has the audacity to cut a piece and put in front you, you just completely ignore it.
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