I totally feel your pain. I have laid out some rules for my brother and sister since I'm at my dad's house sometimes and at school most of the day (though, so is my brother), and I don't want them messing with my pets. My rats are sweet, but one of them was raised with a breeder who fed them through the bars and this background has made her violently snatch and bite (really hard) on ANYTHING that comes through the bars (including my curtains). I tell my brother and sister this every time they come in my room. I am also terrified that I'll come home to 1 or more dead fish because they wanted to feed them. They know that they are never allowed to feed the fish unless I personally hand the pellets to them. I also told them, which I normally would try to protect them from darker subjects, that if they mess with my fish or rats when I'm not around, the animals could and probably WOULD die, and they love my pets almost as much as me
(almost...) They are 7 and 4 btw. You can tell your sis and her bf that if children that young can follow such simple rules, there is no reason they can't either.