Mad at parents

My 15 year old sister texted me and said she wanted to talk. So I met with her & she told me she overheard our parents talking about a weight loss gummy. They wanted to get it for me, but weren't sure if it was good so they apparently said out loud they were going to use my little sister as a guinea pig and if it worked for her to give it to me but otherwise to just not tell me about it.

I can't even believe them. They are obsessed with my weight. Completely unnecessarily, it is none of their business. And now they are going after my little sister. I have FOUGHT my mom about bringing anything up about weight or her body to my sister. She and my dad have wanted to repeatedly and I have said NO. So that they would do something so sketchy like this behind my back is just horrible to me. And for my sister to hear herself being treated like that?? I can't stand my parents. I don't even know what to do about this. What do I even say to them?