Hi Confusedansad,
I can only imagine how painful this situation must be for you. My heart goes out to you and to your grandchildren.
I have a few thoughts.
Since you have identified bedtime as a challenging time of day, you could offer, in a calm moment, that you will do the bedtime routine for a certain number of days per week in order to give her a much needed break, while she takes time for self-care/relaxation time. You could offer to babysit at other times as much as you are willing and able, and to just spend as much time with them as you can manage without wearing yourself out. Taking care of yourself during her meltdowns is very important too. Since you will not be able to reason with her during her outbursts, if the children are not in any physical danger then possibly the best thing for you to do is to leave, even if it's just to go for a walk.
You could ask her if she is open to it any suggestions, and if so you could suggest that she join a parenting support group and learn gentle parenting techniques from a resource such as this:
https://www.ahaparenting.com/ She could also learn about child development so that she can understand what children are capable of at different ages and stages.
This is a very challenging situation and I wish you all the best.