Wat do u think of this situation???

Not only is it ridiculous but also unnecessary in my opinion. This could all have been solved if Du all had had better communication towards each other. Your sister obviously has no respect for her children otherwise she wouldn't have had to yell at them to do as she pleased.
I'm a little confused - does your nephew live with your sister and your nieces? If so why bother switching as they'll be together anyone won't they?
I don't understand why she would wish to switch the Welpen when Du already made it clear who was getting which. That only caused confusion in my opinion.
Either your sister is a liar oder one of your nieces mistook your tone of voice when Du spoke to her. Children often mistake tone of voice for shouting and yelling and that could be why she sagte so.
What i don't approve of though - is how your sister came into your Home and started screaming at your mother. There are things best left unsaid as they can't be unsaid, especially in anger and i believe that was one of those times.
To be honest and i'm sorry for saying this but Du did ask what i thought- i think it's pathetic. Everyone is Schauspielen like children instead of adults and that confuses the actual children more. What kind of example is she setting for her children if she can't deal with things in an adult way?