I have younger children & so far I have to agree with Cailin, consistency.

My daughter is a handful and I have found that if she knows the consequences of doing something wrong, she will think twice about doing it (DD1 who is 3). She often will get frustrated with her new sister getting into her toys and touching things that were hers alone until now. I have encouraged her to vent her frustration and not hold it in.

I get down with Matilda and say "Matilda, I understand its frustrating when Jovie gets into your toys. These toys are to share with her, if you don't want to play with her, take a toy to the lounge & play there"

If that doesn't work I then say, " Matilda, why don't we try to get Jovie interested in something else?" and I take some toys away & try to encourage Jovie to play with them...

if this sparks the end of the world, I tell Matilda "I know its frustrating and hard to learn to share, why don't you have some quiet time to calm down in your bed, if you are angry you can scream into teh pillow to help settled down"

I think its safer for her to scream into a pillow than hit or have to live with the frustrating internalised, ykwim??

And like Cailin, when I find them playing together I praise them both and try to get involved so they can have fun with all of us. I sit down with them & try to do games together so they have the encouragement and I can teach them how to play together. Its hard with one only being 9 months, but I figure if they learn now it will be easier when they are older.