Anyone here ever been home schooled or home schooled their children?
I homeschooled two of my three their entire academic careers from kindergarten through graduation. The youngest has one more year to go.
I wouldn't say anything to your sister at this point. To be honest, it's pretty hard to screw up a 4-yo's pre-school education and a 6 -yo's first grade education. The math, health, reading, art is very simple to teach at that age and the text books quite understandable. One of the benefits of homeschooling is that the teacher gets to refresh or even learn new things they missed in school as they prepare to teach it. If you are that concerned, buy the family The Phonics Game for Christmas. Your sister will actually benefit from playing with them. And encourage your sister to either read to the kids each day or to let them read to her.
And nowadays, homeschooling curriculum had gone very high tech to where the parent never has to teach. One option is satellite classroom where a dish satellite is installed and the kids attend "class" live. The satellite teacher answers questions, grades, etc. Live internet is another option. My youngest took a college level Greek class online through an accredited university and interacted with the professor to get pronunciations correct, ask questions, take tests, etc. I never taught her once (and she made an A in that class, btw). Switched On Schoolhouse is an entire curriculum on CD-ROM requiring only partial parental teaching and oversight. This was our curriculum of choice throughout junior and senior high school.
As they move into high school, there are still other options available. For example, a friend of mine with a degree in Chemistry teaches a homeschool high school level course in chemistry complete with labs. I never taught chemistry. Many public high schools will allow use of their labs for these homeschool classes. Another friend teaches Spanish and another English and yet another Calculus/Trig. These classes do cost money but it's a lot cheaper than private school and the student to teacher ratio is usually something like 4:1 or 8:1. People seem to think homeschoolers are loners but in reality many are members of associations where there is an abundance of competent resources.
I wouldn't say anything to your sister at this point for another reason. Homeschooling isn't for everyone and after a year she may come to that conclusion. At this age, I don't think their educations will be irreparably messed up by one year of homeschooling.