They probably just need clear and consistent boundaries. Raise your expectations of them and let them know what they are.
Food and drinks in the kitchen only.
If they leave their tea and are still hungry, as long as you are happy that the food is something they find palatable they can just have more of their tea. If you're not sure, they could have something like carrot sticks or fruit.
They need to help with making tea, setting the table and clearing away, tidying up toys, etc.
Look for the good things they do and recognise them so they get attention from acceptable behaviour.
Use natural consequences wherever possible.
Show them that you like them but you sometimes don't like their behaviour. Use language like "Yes you can do x as soon as we have finished clearing up." If they don't help clear up, x doesn't happen. Be really consistent with this.
If you don't have a routine, try to build one, e.g:
They get in from school, wash hands and have a snack at the table while you sit with them taking in interest in their day.
Then they do homework if they have it or watch a bit of tv.
The TV goes off and they help make tea.
You sit together to eat and they help clear away.
They then get to go on the x-box until your sister collects them.
Smae routine every day then everyone knows what to expect and they feel contained and secure.