First off, Id tell my husband his responsibility is to his child, not his nieces. Rude, maybe, but that's how I feel about it.
Secondly, like Ingi said, you need to stick with it. Change doesn't come overnight and it may be hard but if you are consistent they will listen.
Think of it as practice for your child.
Also, if you say your going to punish them for whatever they have done, punish them. Do not use empty threats. Just remember your the adult and they are the children. It is for there own good.