--------------------------Miss Emily's advice----------------------
It seems the best way to end it would be for your husband to tell his sister that if he wants her opinion on those matters, he'll ask her -- or to flat out tell her to butt out of your affairs. If he banters with her, this could be a pattern that they developed in childhood (she is probably his older sister?). There could be a chance that he's not totally on-board with the fact you have chosen to home school your son, and other matters, but it would sure be time to find that out! Ask him if he thinks there's any merit to what she says. You won't go ballistic over the truth, but it's better to have a discussion about it than let it fester. If he tells you he's on board with how you handle the home front, then ask him to end the quibbling with his sister over things that are none of her business. If neither of you have the courage to tell her to end her unsolicited criticism over the way you conduct your life, at least ask your husband to keep what she says to himself. You don't need to hear chapter and verse on unsolicited opinions from a know-it-all. Then go on with the life you have a right to lead, and understand that some people are never happier than when they are trying to live other people's lives to avoid dealing with their own.