Sometimes you think to yourself, why do I bother, it's like water off a duck's back. If your DCs are prone to selective hearing, aren't they all it's hard to sound encouraging and upbeat.
Starting on a positive note, praise any and every good action. Lots of parents use a motivational tool, like a star chart. This can cover normal everyday tasks. Sometimes an egg timer to boost their enthusiasm for achieving a chore can help.
When you need to address undesirable behaviour, talk face to face, not from the foot of the stairs or through a wall. For DD2, get down to her height and make eye contact.
Be clever with your tone and body language. Don't raise your voice when cross, keep at a reasonable volume, try a lower tone.
You're not requesting their co-operation like it's optional. Keep it simple and to the point.
Focus on what needs to get done and give an inducement: "When you’ve brushed your teeth, then I’ll read you a story". They might not jump to it if you sound like you are wheedling, so next time phrase it like it's really not optional.
If you feel too many words wash over them, try this especially if pressed for time, use one word like, "Door!" or "PJs!" It doesn't have to sound like you're barking commands, just economical nagging. 
Occasionally you might want to state a fact without accusing: "There’s crumbs on the sofa" and if necessary, explain so they see the point, eg, "When you leave your glass on the floor, it might get kicked over", "When you leave your swimsuit in the bag with the towel, it stays wet and starts to smell".
Sometimes (if it doesn't make more work in the long run for you) let them face consequences. They eventually learn, if they like having choices, they need to sort themselves so their options aren't limited. This applies to forgotten school notes, last minute party invitations suddenly resurfacing, no clean uniform, arranging lifts with friends, bus money, lost gift cards.
Hopefully, if you are consistent things will sink in. 
PS Fwiw I think it's lovely to have a special relationship with another relative, provided the rest of the DCs aren't completely sidelined.