hi P99
The terrible two's aren't called that for nothing LOL - obviously it is different with foster children than birth children as you don't know what they have been through.
For us with our natural children like sumum says pick your battles. As it is all too easy to ask them to do something they say no and then it becomes more about them disobeying what you have asked them to do or not to do.
What works very well for us is time out - we ask the child that is to be disciplined to remove themselves from where we are and then sit on the bottom step, if they are being punished, they have to sit out for 1 minute per year of age. I ask them to think about their behaviour, and when I go out to invite them back in I make sure I first give them a hug and tell them I love them, but did not like their behaviour (when they are older) and then discuss what had happened - for example if one had hit another I would ask them how did they think being hit made the other child feel. If the child is just being unreasonable, or tempers are getting hot, I ask them to go and sit on the time out step, but tell them as soon as they are feeling calm and or ready for a hug to come straight back in. We find that we rarely need to use this.
We find that talking and explaining things to the children really stops most of the need for disipline
such as
ME - can you put your coat on please littlest squidgebrain.
LSB - No
Me - but I would like you to do it, and we need to go and pick up the other squidgetype people
LSB - I don't want to
Me - but if you don't put on your coat you will get cold, and then you might get poorly and I don't want you to be poorly - do you want to get poorly
LSB - No
Me - Oh good then - lets pop on your jacket and then we will head out
LSB - ok then Mummy you know you are a domestic goddess and no your bum doesn't look big in that (OK I might have made that up there......)
As a former paediatric nurse one piece of advice that I would give you, is don't let them off with behaviour that you don't find acceptable just because of their experience, we often had parents who had very ill children, which understandably they were anxious about but would let them really do anything, including physically hurting their siblings but because of their illness they let them away with it, and of course when the child had recovered they were then left with a child that they could no longer discipline them
Good luck with it - and do come back and let us know how you are doing :)