It is difficult to give you advice for your sister. I have not worked with this condition.
When did her diabetes start? How old was she and what was happening in her life then?
What happened in your mother's life when she was carrying your sister?
What is happening now that makes her sugar levels fluctuate?
In Deb Shapiro's book, Healing Mind Healing Body, she says diabetes is an inability to regulate sweetness in one's life, sweetness can be love, too much love (sugar), no ability to process it (insulin), love in and out, love others and oneself....I would start tapping on how she feels now.
Thank you.