We suspect the real problem here is that you are trying to reason and 
be reasonable with a two-year-old, which is not really possible. Two-year- 
olds don’t have the language skills necessary to understand hypothetical 
statements (“If you eat that piece of candy you found on the sidewalk, 
you could get sick”) or to argue with explanations. Moreover, they have 
very little impulse control-they want what they want when they want it. 
This is a normal state that they will outgrow, but for now words will not 
solve the problem. 
     Because your two-year-old is behaving normally, there is no reason 
to punish her or to be angry with her. The real solution is to realize 
explanations are not going to work and solve the problem in a friendly 
but expeditious manner (take the candy and throw it away, strap her into 
her car seat, pick her up and take her away from her sister). Then you 
can comfort her and try to help her engage in an enjoyable activity. You 
will not have the same frustrations. And you will be much less likely to 
lose your temper. Also, keep in mind the goal of anticipating and preventing 
your two-year-old’s meltdowns (have a toy to give her when you put her 
in her car seat, have your older daughter keep her door closed when she 
is doing homework, keep an eye out for attractive garbage and change 
course).