Please don't blame yourself as she is an adult and the job of finding out about diabetes was hers. Now that you are going to look into it more, be assured there's not that much you need to learn about it, to be honest. You are doing great by keeping notes and records, that may help in the future.
The main things you need to know in the situation are: she needs to see her doctor or nurse regularly and have blood tests and checks of other things. Maybe at least every 3 months? Or monthly if things are going badly. She needs to take any meds as directed - plastic containers can help with this if needed. Her eye, kidney and nerve health need to be monitored. Any foot issues or infections anywhere need to be actively managed. Any bladder and bowel issues need to be identified and managed. I think that's the main things but if I've missed anything hopefully others will speak up.
Now might be a good time to get her to sign something giving you and/or your sister enduring power of attorney, in case you end up needing it and for some reason she is reluctant to sign it. This would makes things much easier for you to be able to help her have the best of health, whatever happens.
I think you are doing great - it can be really stressful looking after an elderly parent so remember to look after yourself and schedule regular breaks from it.