I understand how difficult a single mama situation can be. My sister once was put in an even worse position because her roommates, who were watching her then not yet 2 yo DD, thought feeding her tabasco sauce was an appropriate punishment. 
My sister left as soon as she was able, and stopped using her roommates for child care immediately. Of course your situation is with family, which makes it more complicated.
If you're working at least 30 hours a week (I think it's 30), you can probably qualify for state assistance for childcare. And if you look around, you may find that some of the better centers (and licensed in-home providers, who are often very competent and more affordable) take state assistance--part of how they stay in business is passing their exorbiant fees on to the taxpayers. It might hurt your sister's feelings, but then it also might take the stress off of her. If she can't respect your wishes in how your DS is parented and supervised, she shouldn't watch him.

My sister left as soon as she was able, and stopped using her roommates for child care immediately. Of course your situation is with family, which makes it more complicated.
If you're working at least 30 hours a week (I think it's 30), you can probably qualify for state assistance for childcare. And if you look around, you may find that some of the better centers (and licensed in-home providers, who are often very competent and more affordable) take state assistance--part of how they stay in business is passing their exorbiant fees on to the taxpayers. It might hurt your sister's feelings, but then it also might take the stress off of her. If she can't respect your wishes in how your DS is parented and supervised, she shouldn't watch him.








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i'd approach her in a nonconfrontational way and say that she's raised some interesting questions about how you discipline, your options, etc...and that you want to take this opportunity to learn TOGETHER about what is the best course of action. that way it's not "here, sister, here is my knowledge - now go apply it, young squire"...instead, it's "hey, let's learn together about how we can both raise our children in a positive way"
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My DS looks like a humongous, busy giant next to her. Haha. I think my sis is starting to understand more and more about other people "interferring" with parental styles. I asked her just recently (just as an experiment) if she would mind if I spoonfed her baby some whipped cream. She was aghast! She said, "but she's a breastfed baby! She shouldn't get anything but breastmilk!!" Keep in mind she was sneaking "treats" to my Ds at this age. There is hope for my sister, I know this. It is just going to take time, effort, patience, and a lot of trial and error - all things I am learning anyway as a mommy.