Your sister is taking advantage of you and your family. This is so wrong. Yes, it is your sister, however she needs to respect you !
Your kids need to have fun and do things together too! If this is compromising your kids and DH , then enough is enough . If you do not put your foot down then she will continue to use you.
I know you love your niece, and she is probably bonded with you. However, Your sister seems to make it up to her kids , and is not as financially strapped as it appears.
She must think you are not very smart or care, because to me, she is using you as well as manipulating you.
I am sorry