Teenager rant!

Just need to rant about teenage SD15 and get the rage off my chest! Our SD lives full time with us and has done for 5 years now. We have a great family dynamic just usual teenager ups and downs but recently with my baby, 9 months, she is driving me crazy. Multiple times I’ve asked her not to feed baby and to ask if sharing food, I’ve seen her sneak him sugary cereal, sips of her drinks, it drives me crazy! I know it’s small bits and I might sound too controlling, but she has terrible teeth and way too much sugar, and is literally having to have teeth pulled out this week. This is totally not her fault and one of the reasons she does not live with her BM anymore, so I think controlling the sugar for baby is even more important because of this! This morning I gave baby some banana and quickly popped some washing on, (left with dad in kitchen) when I came back she was giving him a frube, I said to Dad did you ok that? He said no. So I said please don’t feed the baby it’s vital we look after his new teeth at the moment. Her and Dad just looked miffed at me. Am I being too controlling?! I feel like I need to address it with DH because right now feel like I can’t leave them alone to look after baby, but I’m fighting a losing battle and will cause argument with Dad as ultimately he is responsible for her teeth as well and I don’t trust either of them when it comes to sugar intake for baby!