Some of the posters in this thread have been on here for many years. Some, I would say over a decade. And we can confirm that what you stated is perceived as working. There have been posts like yours of people visiting and helping their family with kids in the US only to be later denied entry or having their visa revoked for working.
Your sister did not have to be directly paid by you for it to be considered working. In fact, if you think about it, you provided her with room and board for half a year. That can be perceived as a form of payment in lieu of money.
America is very expensive to live in. And I would even say it was very American of you to seek cheaper options for something that is pricey. However, when it comes to immigration, you have to make sure you know all the rules and nuances involved.
I agree that your sister will likely not get another visa, at least for the time being. She can still get an immigrant visa later if she chooses to want to live in the US. However, it will take about 15 years to complete.
Please continue to use this forum for any other question or concerns you may have. Many of us can say we have just about seen it all. If you have other family that are visiting, feel free to ask about common Dos and Don'ts.