The key thing is to pick your battles, be consistent and be logical. There are few things more logical than a kid.
Nobody really likes being told what to do, and at 5 your sister is probably trying to find out who she is and what she likes, which involves experimentation. One of the easiest and fastest ways to realise that it's impossible to play in the garden in white trousers without getting them filthy is to try it, for example. Obviously this doesn't extent to finding out how hot boiling water feels though.
I see kids at the park who are constantly having instructions barked at them - ' don't do that. do this. not like that. come here. we're going home' and all that. Obviously they ignore their parents.
Yesterday there was a kid on one of the pedalos on the boating lake, and - you'll never guess - he got excited, pedalled quite hard and got water in the boat, so his shoes got wet. Imagine - a kid enjoying themselves on water getting wet! Parental reaction: "Bloody hell you ruin everything. We're going home - come on!" Kid "I don't want to go home, mum" - they ended up staying. Parent loses respect because they over-reacted and backed down, but loses less respect than if they'd over-reacted and not backed down I suppose.
Anyway, I'm not answering the question am I? We use the phrase "This is non-negotiable" when it's non-negotiable. The kid knows what it means, and knows we mean it because we've never negotiated anything we've declared non-negotiable, and because we hardly ever say it. Crossing the road without holding hands is one, and until yesterday she has never questioned it at all. Yesterday she asked how old she needs to before she can cross without "doing holdy-hands" DP told her 10.
Kids will do things you would rather they didn't, but unless it's harmful to her or bothersome to anyone else, we let her get on with it, although we might tell her why we'd rather she didn't or try and get her to do something else.