Love your Title!
Maybe it bothers you so intensely because it is your sister. If it was a random stranger, you could more easily chalk it up to none of my business. With a loved one, it just hurts so much. Hugs.
Like you said though, she loves her DC just as much as you love yours, she's just got different ways. I think the car seat I'd just make my peace and then hope there is never an accident, that is all you can do. And the food, well, you can't do much about that either. And the CIO, you cant do anything about it when they are not there, but then again you won't hear it, so you can put it out of your mind. But do think WHILE they are in your house, you could just say "it hurts so much, I can't let him cry alone like that" and then go pick him up? Then you've become the fall guy, blamed your emotions on yourself, but you haven't asked her to pick up her own baby, you do it. That way you give yourself some peace, and he will get some love and stop screaming. And even maybe she will see, by your example, that the sky will not cave in just because she comforts her child.