family member asked to stay at my home again

ugh!!!!
I can't say no, not to my neph and neice, to the adults yes, but to my lil neph and niece, I can't.
so i said "yes, but only to stay in the living room and they can only be there when I am there, and when I am gone they have to be gone." 
I don't know how this is going to work, they said groceries, and light bill, but last time, they didn't do anything they promised and I end up taking care of their children and my kids end up babysitting 99.9 % of the time. 
Ugh!!!! I hope they find another place soon. I am tired of people always asking me to board them and they don't help me out in any way shape of form. 
Ugh!!! 
why cant i just say "NO"