Dd dominates ds, how can I stop her?

[Kelly1978]

I have a dd who is 4 and a ds who is 2. Dd has always been very confident and outgoing and totally dominates her quieter brother. It's the first day of half term and she is driving me nuts because ds keeps coming thru crying because she has done soemthing or other to him. They love each other to bits, insist on playing together but it always ends in tears!
Ds is a slower developer for his age, doesn't talk much and doesn't seem to be able to stand up to her.
Any ideas on how to encourage their realtionship to balance otu a bit wouldbe really appreciated!