I wish you hadn't helped her although I can understand why you did. You aren't wrong for being offended. Doesn't matter how many times a person does it,being unappreciated sucks. You should probably start distancing yourself from her. And if she asks why, tell her. She starts causing a scene ignore it. Explain you've said what needs saying and that's all. I do realise its easier said than done. But you have every right to be upset.
Think that is good advice, only problem is I know my nieces would miss me.
At the end of the day your sister would get over it. I'm not saying cut her out of your life. I'm just saying she needs to know how she is. She'll be harsh for a bit I'm sure,but bc your nieces would miss you it wouldn't last long. Again, I know its much easier said than done.