My grandson was 2 on Thursday, and a joy to have around, but oh, am I tired after a day looking after him?!! And there's only one of him and I have the back up of my husband, who he adores.
This is difficult for you. You absolutely adore these little boys and they you. You love them unconditionally as they deserve. This is the greatest gift anyone can give a child. They will grow up knowing they are loveable and will return the gift to others. Many people never experience this.
I gather your relationship with your sister is not what you would like. I suspect she will be on the defensive at any hint of criticism.
People tend to think discipline equals punishment. To me, as both a parent and as a teacher, discipline is part a loving relationship. Yes, there are times when you have to say no, but it can be said firmly with love. Shouting at anyone in anger will either be ignored or obeyed out of fear and resentment. ( we all have our moments of anger) With children, I had to decide what behaviour I wanted, then praised any behaviour that approximated what I was aimed at. Try getting a class of 4 year olds to tidy up by shouting. It won't happen, but if you praise one child for putting something away, then they all get the idea.
The same applies to adults. How do you want your sister to be with her boys? If you see any glimmer of that behaviour and comment how lovely the boys were when she said or did....... I have done this with my husband. ( please no one tell him!). He used to do little in the house. I decided what would help me when I wasn't well, praised and thanked him whenever he did anything. (even when I have had to redo the task!). Basically tell her where she is going right not where she is going wrong.
Do you feel any love and respect for your sister? It will help if you can find some glimmer, however small. It's not a guarantee, but relationships can change if you change and think about your responses. However, if she really doesn't give a damn, you can't help. You can't help someone if they don't want help.
I don't know whether this helps at all, but you need to feel easier about the boys so you can take some steps away. Love Ju Juxx